diff --git a/backend/services/planning_engine.py b/backend/services/planning_engine.py
index 7925fe1..54b997f 100644
--- a/backend/services/planning_engine.py
+++ b/backend/services/planning_engine.py
@@ -613,6 +613,11 @@ def solve_dispatch(
                 + heat_pump.rated_heating_power_w
             )
 
+        # Záporný prodej (sell < 0): nevybíjet baterii do sítě pro arbitráž.
+        # Export v tomto okně může vzniknout jen z přebytku FVE (pv_a/pv_b), ne z bd.
+        if s.sell_price < 0:
+            prob += w_arb[t] == 0
+
         soc_prev_expr = current_soc_wh if t == 0 else soc[t - 1]
         arb_t = arb_floor_series[t]
         soc_low_t = soc_panel_min[t]
diff --git a/backend/tests/test_planning_dispatch_milp.py b/backend/tests/test_planning_dispatch_milp.py
index 62209bb..3a718c1 100644
--- a/backend/tests/test_planning_dispatch_milp.py
+++ b/backend/tests/test_planning_dispatch_milp.py
@@ -439,6 +439,84 @@ class PlanningDispatchMilpTests(unittest.TestCase):
                 msg="with relaxed soc_min, first-slot export should be able to finish below reserve %",
             )
 
+    def test_negative_sell_forbids_battery_export_arbitrage(self) -> None:
+        """
+        Pokud sell < 0, solver nesmí vybíjet baterii do sítě pro arbitráž (dump musí proběhnout předtím).
+        V okně sell<0 smí export vzniknout jen z přebytku FVE; zde ale FVE=0, takže očekáváme grid_setpoint>=0.
+        """
+        base = datetime(2026, 4, 3, 6, 0, tzinfo=timezone.utc)
+        s0 = PlanningSlot(
+            interval_start=base,
+            buy_price=2.0,
+            sell_price=2.0,
+            pv_a_forecast_w=0,
+            pv_b_forecast_w=0,
+            load_baseline_w=0,
+            ev1_connected=False,
+            ev2_connected=False,
+            is_predicted_price=False,
+            allow_charge=True,
+            allow_discharge_export=True,
+        )
+        s1 = PlanningSlot(
+            interval_start=base + timedelta(minutes=15),
+            buy_price=2.0,
+            sell_price=-0.5,
+            pv_a_forecast_w=0,
+            pv_b_forecast_w=0,
+            load_baseline_w=0,
+            ev1_connected=False,
+            ev2_connected=False,
+            is_predicted_price=False,
+            allow_charge=True,
+            allow_discharge_export=True,
+        )
+        s2 = PlanningSlot(
+            interval_start=base + timedelta(minutes=30),
+            buy_price=-15.0,
+            sell_price=-1.0,
+            pv_a_forecast_w=0,
+            pv_b_forecast_w=0,
+            load_baseline_w=0,
+            ev1_connected=False,
+            ev2_connected=False,
+            is_predicted_price=False,
+            allow_charge=True,
+            allow_discharge_export=True,
+        )
+        slots = [s0, s1, s2]
+        battery = _battery(uc_wh=10_000.0, min_pct=10.0, arb_pct=20.0)
+        battery.planner_extreme_buy_threshold_czk_kwh = -2.0
+        battery.planner_discharge_floor_percent = 5.0
+        battery.max_charge_power_w = 50_000
+        battery.max_discharge_power_w = 50_000
+        hp = SimpleNamespace(
+            rated_heating_power_w=0,
+            tuv_min_temp_c=45.0,
+            tuv_target_temp_c=55.0,
+        )
+        grid = SimpleNamespace(max_import_power_w=50_000, max_export_power_w=50_000)
+        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),
+        ]
+        soc0 = 0.9 * battery.usable_capacity_wh
+        results, _ms = solve_dispatch(
+            slots,
+            battery,
+            hp,
+            grid,
+            [None, None],
+            vehicles,
+            soc0,
+            50.0,
+            tuv_delta_stats=None,
+            operating_mode="AUTO",
+        )
+        self.assertEqual(len(results), 3)
+        # V sell<0 slotu bez FVE a bez zátěže nesmí být export (to by muselo být z baterie).
+        self.assertGreaterEqual(results[1].grid_setpoint_w, 0)
+
     def test_grid_import_soft_cap_penalizes_breaker_overdraw(self) -> None:
         """
         Soft cap: solver může nominálně překročit breaker, ale jen pokud se to vyplatí.