diff --git a/backend/services/planning_engine.py b/backend/services/planning_engine.py
index a0a92c9..d72fac9 100644
--- a/backend/services/planning_engine.py
+++ b/backend/services/planning_engine.py
@@ -59,7 +59,7 @@ NEG_SELL_CURTAIL_PENALTY_CZK_KWH = 1.0
 NEG_SELL_PV_CHARGE_REWARD_CZK_KWH = 0.8
 # Měkký tlak: v okně sell<0 dobít na soc_max (ne zastavit na ~94 % kvůli curtail).
 NEG_SELL_SOC_UNDERFILL_PENALTY_CZK_PER_WH = 0.35
-PLANNER_BUILD_TAG = "2026-05-25-neg-sell-no-export-fixed-v8"
+PLANNER_BUILD_TAG = "2026-05-25-purchase-fixed-neg-sell-v9"
 CORRECTION_WINDOW_H  = 1        # hodina zpět pro výpočet korekčního faktoru
 CORRECTION_MIN_CLAMP = 0.5      # spodní limit korekčního faktoru
 CORRECTION_MAX_CLAMP = 1.5      # horní limit korekčního faktoru
@@ -665,11 +665,20 @@ def _slot_profitable_battery_export(
     return sell_t > acq + min_spread
 
 
+def _purchase_pricing_fixed(grid: Any) -> bool:
+    """Režim nákupu z DB (`site_market_config.purchase_pricing_mode`), ne odhad z rozptylu buy."""
+    return (
+        str(getattr(grid, "purchase_pricing_mode", "spot") or "spot").strip().lower()
+        == "fixed"
+    )
+
+
 def _horizon_fixed_tariff_like(slots: list[PlanningSlot]) -> bool:
     """
-    Fixní nákup (KV1): buy v horizontu je prakticky konstantní.
+    Heuristika pro drahý import / charge_acquisition: buy v horizontu je prakticky konstantní.
     U spotu (home-01) nesmí expensive_import používat charge_acquisition — jinak
     buy > ~1 Kč označí téměř všechny sloty jako drahé (gi=0 pro dům) → Infeasible.
+    BA81 má fixní nákup v DB, ale NT/VT → buy skáče; proto neg-sell export řídí _purchase_pricing_fixed.
     """
     buys = [float(s.buy_price) for s in slots if float(s.buy_price) >= 0.0]
     if not buys:
@@ -1184,7 +1193,8 @@ def solve_dispatch(
         else min(float(s.buy_price) for s in slots)
     )
     min_spread_pre = float(degradation_cost_effective)
-    fixed_tariff_like_pre = _horizon_fixed_tariff_like(slots)
+    purchase_fixed_pre = _purchase_pricing_fixed(grid)
+    fixed_tariff_like_pre = purchase_fixed_pre or _horizon_fixed_tariff_like(slots)
     profitable_export_ts_pre: set[int] = set()
     if om == "AUTO":
         for _t in range(T):
@@ -1641,8 +1651,9 @@ def solve_dispatch(
                 prob += ge[t] == 0
                 prob += ge_pv[t] == 0
                 prob += ge_bat[t] == 0
-            elif fixed_tariff_like_pre:
-                # BA81: při sell<0 neexportovat (záporná cena výkupu = platíš za export).
+            elif purchase_fixed_pre:
+                # Fixní nákup + spot výkup (BA81, KV1 bez block_export): sell<0 = platíš za vývoz.
+                # Nesouvisí s NT/VT skokem buy — řídí se výkupní cenou, ne rozptylem buy v horizontu.
                 # Přebytek FVE → baterie / curtail A; B přes z_gen_cutoff nebo bc_pv.
                 prob += ge[t] == 0
                 prob += ge_pv[t] == 0
@@ -1833,7 +1844,7 @@ def solve_dispatch(
                 float(s.pv_b_forecast_w) > 0
                 and not getattr(grid, "block_export_on_negative_sell", False)
                 and sell_t < 0
-                and not fixed_tariff_like_pre
+                and not purchase_fixed_pre
             ) or (
                 # KV1: plná baterie + kladný sell — neblokovat ge_pv==0 (jinak masivní curtail).
                 getattr(grid, "block_export_on_negative_sell", False)
@@ -1842,7 +1853,7 @@ def solve_dispatch(
             )
             # BA81: export pole B jen při kladném sell (po sell<0 jinak ge==0 výše).
             fixed_pv_b_export_cap = (
-                fixed_tariff_like_pre
+                purchase_fixed_pre
                 and float(s.pv_b_forecast_w) > 0
                 and not getattr(grid, "block_export_on_negative_sell", False)
                 and sell_t >= 0
@@ -1870,7 +1881,7 @@ def solve_dispatch(
             # Spot (home-01): buy > min ne-záporného buy v horizontu.
             # Fixní tarif (KV1): navíc buy > charge_acquisition (konstantní buy ≈ ref).
             expensive_import_slot = buy_t > ref_buy_horizon + min_spread
-            if _horizon_fixed_tariff_like(slots):
+            if fixed_tariff_like_pre:
                 expensive_import_slot = expensive_import_slot or (
                     buy_t > charge_acquisition_czk_kwh + min_spread
                 )
@@ -2600,11 +2611,14 @@ async def _load_site_context(site_id: int, db):
     )
 
     g = ctx["grid"]
+    m = ctx.get("market") or {}
     grid = SimpleNamespace(
         max_import_power_w=int(g["max_import_power_w"]),
         max_export_power_w=int(g["max_export_power_w"]),
         block_export_on_negative_sell=bool(g.get("block_export_on_negative_sell") or False),
         deye_gen_microinverter_cutoff_enabled=bool(g.get("deye_gen_microinverter_cutoff_enabled") or False),
+        purchase_pricing_mode=str(m.get("purchase_pricing_mode") or "spot").strip().lower(),
+        sale_pricing_mode=str(m.get("sale_pricing_mode") or "spot").strip().lower(),
     )
 
     vehicles: list[SimpleNamespace] = []
diff --git a/backend/tests/test_planning_dispatch_milp.py b/backend/tests/test_planning_dispatch_milp.py
index 2c7a91c..2eace9b 100644
--- a/backend/tests/test_planning_dispatch_milp.py
+++ b/backend/tests/test_planning_dispatch_milp.py
@@ -1222,7 +1222,7 @@ class NegativeSellPvChargeTests(unittest.TestCase):
             50.0,
             operating_mode="AUTO",
         )
-        self.assertEqual(snap.get("planner_build_tag"), "2026-05-25-neg-sell-no-export-fixed-v8")
+        self.assertEqual(snap.get("planner_build_tag"), "2026-05-25-purchase-fixed-neg-sell-v9")
         self.assertGreater(
             results[0].battery_setpoint_w,
             5_500,
@@ -1366,7 +1366,7 @@ class NegativeSellPvChargeTests(unittest.TestCase):
             50.0,
             operating_mode="AUTO",
         )
-        self.assertEqual(snap.get("planner_build_tag"), "2026-05-25-neg-sell-no-export-fixed-v8")
+        self.assertEqual(snap.get("planner_build_tag"), "2026-05-25-purchase-fixed-neg-sell-v9")
         self.assertEqual(len(results), len(slots))
 
     def test_gen_cutoff_full_soc_neg_sell_with_pv_b_feasible(self) -> None:
@@ -1430,7 +1430,7 @@ class NegativeSellPvChargeTests(unittest.TestCase):
             55.0,
             operating_mode="AUTO",
         )
-        self.assertEqual(snap.get("planner_build_tag"), "2026-05-25-neg-sell-no-export-fixed-v8")
+        self.assertEqual(snap.get("planner_build_tag"), "2026-05-25-purchase-fixed-neg-sell-v9")
         self.assertEqual(len(results), len(slots))
 
     def test_fixed_tariff_neg_sell_no_grid_export(self) -> None:
@@ -1460,6 +1460,8 @@ class NegativeSellPvChargeTests(unittest.TestCase):
             max_export_power_w=16_000,
             block_export_on_negative_sell=False,
             deye_gen_microinverter_cutoff_enabled=True,
+            purchase_pricing_mode="fixed",
+            sale_pricing_mode="spot",
         )
         hp = SimpleNamespace(
             rated_heating_power_w=0,
@@ -1493,6 +1495,71 @@ class NegativeSellPvChargeTests(unittest.TestCase):
             self.assertGreaterEqual(r.battery_setpoint_w, 0, "neg sell má nabíjet")
             self.assertGreaterEqual(r.grid_setpoint_w, 0, "neg sell bez exportu do sítě")
 
+    def test_ba81_fixed_purchase_nt_vt_buy_spread_neg_sell_no_export(self) -> None:
+        """BA81: NT/VT buy v horizontu (rozptyl >0,25) — záporný sell stále bez exportu."""
+        slots = [
+            PlanningSlot(
+                interval_start=datetime(2026, 5, 25, 7, 0, tzinfo=timezone.utc)
+                + timedelta(minutes=15 * i),
+                buy_price=3.088 if i % 2 == 0 else 4.086,
+                sell_price=-0.5,
+                pv_a_forecast_w=10_000,
+                pv_b_forecast_w=2_500,
+                load_baseline_w=300,
+                ev1_connected=False,
+                ev2_connected=False,
+                allow_charge=True,
+                allow_discharge_export=False,
+                charge_acquisition_buy_czk_kwh=3.61,
+            )
+            for i in range(4)
+        ]
+        self.assertGreater(
+            max(s.buy_price for s in slots) - min(s.buy_price for s in slots),
+            0.25,
+        )
+        battery = _battery(uc_wh=12_500.0, terminal_soc_value_factor=0.0)
+        battery.soc_max_wh = 12_500.0
+        battery.max_charge_power_w = 6_250
+        grid = SimpleNamespace(
+            max_import_power_w=17_000,
+            max_export_power_w=16_000,
+            block_export_on_negative_sell=False,
+            deye_gen_microinverter_cutoff_enabled=True,
+            purchase_pricing_mode="fixed",
+            sale_pricing_mode="spot",
+        )
+        hp = SimpleNamespace(
+            rated_heating_power_w=0,
+            tuv_min_temp_c=45.0,
+            tuv_target_temp_c=55.0,
+        )
+        vehicles = [
+            SimpleNamespace(
+                max_charge_power_w=0,
+                battery_capacity_kwh=1.0,
+                default_target_soc_pct=80.0,
+            ),
+            SimpleNamespace(
+                max_charge_power_w=0,
+                battery_capacity_kwh=1.0,
+                default_target_soc_pct=80.0,
+            ),
+        ]
+        results, _ms, _ = solve_dispatch(
+            slots,
+            battery,
+            hp,
+            grid,
+            [None, None],
+            vehicles,
+            8_000.0,
+            50.0,
+            operating_mode="AUTO",
+        )
+        for r in results:
+            self.assertGreaterEqual(r.grid_setpoint_w, 0)
+
 
 class AutoPvSurplusExportTests(unittest.TestCase):
     """Plná baterie + vysoká FVE: export přebytku (ge_pv), ne curtailment, bez SELL."""
diff --git a/db/routines/R__039_fn_planning_site_context.sql b/db/routines/R__039_fn_planning_site_context.sql
index 2f8334e..2f7e3f4 100644
--- a/db/routines/R__039_fn_planning_site_context.sql
+++ b/db/routines/R__039_fn_planning_site_context.sql
@@ -10,6 +10,7 @@ declare
   v_b jsonb;
   v_hp jsonb;
   v_grid jsonb;
+  v_market jsonb;
   v_veh jsonb;
   v_ev jsonb;
   v_soc_pct numeric;
@@ -136,6 +137,25 @@ begin
     raise exception 'No site_grid_connection for site_id=%', p_site_id;
   end if;
 
+  select jsonb_build_object(
+    'purchase_pricing_mode', lower(trim(coalesce(smc.purchase_pricing_mode, 'spot'))),
+    'sale_pricing_mode', lower(trim(coalesce(smc.sale_pricing_mode, 'spot')))
+  )
+  into v_market
+  from ems.site_market_config smc
+  where smc.site_id = p_site_id
+    and smc.valid_to is null
+  order by smc.valid_from desc
+  limit 1;
+
+  v_market := coalesce(
+    v_market,
+    jsonb_build_object(
+      'purchase_pricing_mode', 'spot',
+      'sale_pricing_mode', 'spot'
+    )
+  );
+
   select coalesce(
     jsonb_agg(
       jsonb_build_object(
@@ -263,6 +283,7 @@ begin
     'battery', v_b,
     'heat_pump', v_hp,
     'grid', v_grid,
+    'market', v_market,
     'vehicles', v_veh,
     'ev_sessions', v_ev,
     'soc_wh', v_soc_wh,
diff --git a/docs/04-modules/planning.md b/docs/04-modules/planning.md
index 0539cee..0c11d37 100644
--- a/docs/04-modules/planning.md
+++ b/docs/04-modules/planning.md
@@ -18,7 +18,8 @@
   - **Load-first (Deye, AUTO):** proměnné `pv_ld` (PV → load+EV+TČ), `pv_sp` (přebytek), `bc_pv` / `bc_gi`. Plná bilance `pv_a + pv_b + gi + bd = load + ev + hp + bc + ge`; `bc_pv + ge_pv ≤ pv_sp`; `gi ≤ load + bc_gi`; mimo `allow_discharge_export`: `bd ≤ load − pv_ld` a **`pv_ld ≥ load − gi − bd`**. Snapshot: `load_first_enabled=true`. Test `LoadFirstDispatchTests`.
   - **Tvrdé výkonové limity site/baterie:** `gi ≤ site_grid_connection.max_import_power_w` (breaker); **`bc_pv + bc_gi ≤ asset_battery.max_charge_power_w`**; **`ge ≤ max_export_power_w`** (proměnná `ge`, platí `ge = ge_pv + ge_bat`); **`bd + ge_bat ≤ asset_battery.max_discharge_power_w`** (vybíjení do domu + export z baterie nesmí současně překročit BMS). Dříve LP dovoloval import+nabíjení a dvojnásobné nabíjení; u prodeje hrozilo současné `bd` a `ge_bat` až 2× max discharge — viz `SitePowerCapTests`.
   - **Hodnota FVE (PV store value):** `ge_pv = 0`, pokud `sell < future_sell_opportunity − degradation` (ne `charge_acquisition` — u fixního KV1 by jinak blokoval export při sell 2 Kč). **Před prvním `sell < 0` v horizontu:** při `sell ≥ 0` smí `ge_pv` až do `pv_sp` (strategie BA81: vyvézt přes poledne, pak nabít z FVE v záporném okně). Výjimka **nucený vent** jen plná baterie. Testy `Home01PvStoreValueTests`, `PreNegativeSellExportTests`.
-  - **Drahý nákup → vlastní spotřeba z baterie:** mimo `allow_charge` platí `bd + pv_ld ≥ load_baseline + hp[t]` a `gi ≤ EV + hp[t]` (ne `hp_rated`). **Spot:** drahý slot = `buy > min(buy≥0) + degradace`. **Fixní tarif (KV1):** navíc `buy > charge_acquisition + degradace` (`_horizon_fixed_tariff_like` — rozptyl buy &lt; 0,25 Kč/kWh). Na spotu **nesmí** `charge_acquisition` (~0,9 Kč) označit všechny sloty jako drahé → Infeasible (home-01). Při **Infeasible** solver jednou opakuje s `relaxed_expensive_import` (síť smí krmit baseload v drahých slotech; v `solver_params.inputs.relaxed_expensive_import=true`). Testy `AutoPassiveSelfConsumptionTests`, `test_spot_low_acquisition_does_not_mark_all_slots_expensive`, `test_negative_buy_in_horizon_does_not_block_all_grid_import`.
+  - **Drahý nákup → vlastní spotřeba z baterie:** mimo `allow_charge` platí `bd + pv_ld ≥ load_baseline + hp[t]` a `gi ≤ EV + hp[t]` (ne `hp_rated`). **Spot:** drahý slot = `buy > min(buy≥0) + degradace`. **Fixní nákup (DB `purchase_pricing_mode=fixed` nebo heuristika rozptylu buy &lt; 0,25):** navíc `buy > charge_acquisition + degradace`. Na spotu **nesmí** `charge_acquisition` (~0,9 Kč) označit všechny sloty jako drahé → Infeasible (home-01). Při **Infeasible** solver jednou opakuje s `relaxed_expensive_import` (síť smí krmit baseload v drahých slotech; v `solver_params.inputs.relaxed_expensive_import=true`). Testy `AutoPassiveSelfConsumptionTests`, `test_spot_low_acquisition_does_not_mark_all_slots_expensive`, `test_negative_buy_in_horizon_does_not_block_all_grid_import`.
+  - **Záporný výkup (`sell < 0`) bez exportu:** `block_export_on_negative_sell` (KV1) **nebo** `purchase_pricing_mode=fixed` z `fn_planning_site_context` (BA81) — **ne** podle rozptylu buy v horizontu (v8 chyba). Spot nákup (home-01) může při plné baterii ventovat PV B. Tag `2026-05-25-purchase-fixed-neg-sell-v9`, test `test_ba81_fixed_purchase_nt_vt_buy_spread_neg_sell_no_export`.
   - **Pole B při sell&lt;0 (home-01):** pokud `block_export_on_negative_sell = false`, LP nesmí vynutit `ge_pv = 0` (přebytek neriťitelného PV B). KV1 s `block_export = true` jen curtail A / nabíjení.
   - **`ref_buy_min` (brána exportu):** `min(buy_price)` horizontu — jen „existuje levný nákup?“, **ne** průměrná cena nabití přes hodiny. Export sloty: `sell > ref_buy_min + degradation` (spot). Viz [`planning-arbitrage-accounting.md`](planning-arbitrage-accounting.md).
   - Pokud `energy_to_fill <= 0` nebo `charge_slot_buffer = 0`: všechny sloty povoleny.
diff --git a/docs/planning-changelog.md b/docs/planning-changelog.md
index 8d62c54..a9f0764 100644
--- a/docs/planning-changelog.md
+++ b/docs/planning-changelog.md
@@ -5,6 +5,20 @@ Formát: **datum (ISO)** · stručný důvod · soubory · chování / ověřen
 
 ---
 
+## 2026-05-25 (m) — BA81: záporný výkup bez exportu podle DB `purchase_pricing_mode`
+
+**Problém (tag v8 v produkci):** KV1 OK; **BA81** pořád export při `sell < 0` (dnes i zítra). v8 používalo `_horizon_fixed_tariff_like` (rozptyl **buy** &lt; 0,25 Kč/kWh). U BA81 buy skáče **NT/VT** (3,09 ↔ 4,09) → heuristika **false** → zákaz exportu se neaplikoval.
+
+**Oprava (tag `2026-05-25-purchase-fixed-neg-sell-v9`):**
+
+- `ems.fn_planning_site_context` vrací **`market.purchase_pricing_mode`** / **`sale_pricing_mode`** z `site_market_config`.
+- Při **`sell < 0`** a **`purchase_pricing_mode = fixed`**: `ge = 0` (nezávisle na rozptylu buy). **home-01** (spot nákup) výjimku nemá — může ventovat PV B.
+- `_horizon_fixed_tariff_like` zůstává jen pro **drahý import** / `charge_acquisition` (heuristika + DB `fixed`).
+
+**Ověření:** `pytest …::NegativeSellPvChargeTests::test_ba81_fixed_purchase_nt_vt_buy_spread_neg_sell_no_export`; po deploy + replan BA81: žádný `grid < 0` při `sell < 0` v MCP.
+
+---
+
 ## 2026-05-25 (l) — Plán 25. 5.: BA81 neg. výkup bez exportu, KV1 ranní curtail
 
 **Problém (MCP plán run 16346–16350, tag v7):** KV1 06–08 h masivní **curtail** FVE (plná baterie, `ge_pv=0` z pv_store). BA81 při `sell<0` **export ~10 kW** místo nabíjení. Večer slabý export u KV1/home-01 (spot: `sell < buy`).