Step 6: Update exchange creation logic to use new pricing config

- Update ExchangeService to load pricing config from database
- Update validate_eur_amount to use direction-specific limits
- Update apply_premium_for_direction to calculate base + extra premium
- Update create_exchange to use new premium calculation
- Add tests for premium calculation (small trade extra, large trade base only, direction-specific)
- Update existing tests to account for new premium calculation

backend/services/exchange.py

@@ -25,13 +25,11 @@ from models import (
 from repositories.availability import AvailabilityRepository
 from repositories.exchange import ExchangeRepository
 from repositories.price import PriceRepository
+from repositories.pricing import PricingRepository
 from schemas import AvailableSlotsResponse, BookableSlot
 from shared_constants import (
     EUR_TRADE_INCREMENT,
-    EUR_TRADE_MAX,
-    EUR_TRADE_MIN,
     LIGHTNING_MAX_EUR,
-    PREMIUM_PERCENTAGE,
     PRICE_STALENESS_SECONDS,
     SLOT_DURATION_MINUTES,
 )
@@ -48,24 +46,53 @@ class ExchangeService:
         self.price_repo = PriceRepository(db)
         self.exchange_repo = ExchangeRepository(db)
         self.availability_repo = AvailabilityRepository(db)
+        self.pricing_repo = PricingRepository(db)
 
-    def apply_premium_for_direction(
+    async def apply_premium_for_direction(
         self,
         market_price: float,
-        premium_percentage: int,
+        eur_amount: int,
         direction: TradeDirection,
-    ) -> float:
+    ) -> tuple[float, int]:
         """
-        Apply premium to market price based on trade direction.
+        Apply premium to market price based on trade direction and amount.
 
         The premium is always favorable to the admin:
         - When user BUYS BTC: user pays MORE (market * (1 + premium/100))
         - When user SELLS BTC: user receives LESS (market * (1 - premium/100))
+
+        Premium calculation:
+        - Base premium for direction (from pricing config)
+        - Add extra premium if trade amount <= small trade threshold
+
+        Returns:
+            Tuple of (agreed_price, total_premium_percentage)
         """
+        # Load pricing config
+        pricing_config = await self.pricing_repo.get_current()
+        if pricing_config is None:
+            raise ServiceUnavailableError("Pricing configuration not available")
+
+        # Get base premium for direction
+        base_premium = (
+            pricing_config.premium_buy
+            if direction == TradeDirection.BUY
+            else pricing_config.premium_sell
+        )
+
+        # Calculate total premium (base + extra if small trade)
+        if eur_amount <= pricing_config.small_trade_threshold_eur:
+            total_premium = base_premium + pricing_config.small_trade_extra_premium
+        else:
+            total_premium = base_premium
+
+        # Apply premium to market price
         if direction == TradeDirection.BUY:
-            return market_price * (1 + premium_percentage / 100)
+            agreed_price = market_price * (1 + total_premium / 100)
         else:  # SELL
-            return market_price * (1 - premium_percentage / 100)
+            agreed_price = market_price * (1 - total_premium / 100)
+
+        return agreed_price, total_premium
 
     def calculate_sats_amount(
         self,
@@ -147,12 +174,39 @@ class ExchangeService:
 
         return cached_price
 
-    async def validate_eur_amount(self, eur_amount: int) -> None:
-        """Validate EUR amount is within configured limits."""
-        if eur_amount < EUR_TRADE_MIN * 100:
-            raise BadRequestError(f"EUR amount must be at least €{EUR_TRADE_MIN}")
-        if eur_amount > EUR_TRADE_MAX * 100:
-            raise BadRequestError(f"EUR amount must be at most €{EUR_TRADE_MAX}")
+    async def validate_eur_amount(
+        self, eur_amount: int, direction: TradeDirection
+    ) -> None:
+        """Validate EUR amount is within configured limits for the given direction."""
+        # Load pricing config
+        pricing_config = await self.pricing_repo.get_current()
+        if pricing_config is None:
+            raise ServiceUnavailableError("Pricing configuration not available")
+
+        # Get direction-specific limits
+        eur_min = (
+            pricing_config.eur_min_buy
+            if direction == TradeDirection.BUY
+            else pricing_config.eur_min_sell
+        )
+        eur_max = (
+            pricing_config.eur_max_buy
+            if direction == TradeDirection.BUY
+            else pricing_config.eur_max_sell
+        )
+
+        if eur_amount < eur_min:
+            direction_str = direction.value.upper()
+            raise BadRequestError(
+                f"EUR amount must be at least €{eur_min / 100:.0f} "
+                f"for {direction_str} trades"
+            )
+        if eur_amount > eur_max:
+            direction_str = direction.value.upper()
+            raise BadRequestError(
+                f"EUR amount must be at most €{eur_max / 100:.0f} "
+                f"for {direction_str} trades"
+            )
         if eur_amount % (EUR_TRADE_INCREMENT * 100) != 0:
             raise BadRequestError(
                 f"EUR amount must be a multiple of €{EUR_TRADE_INCREMENT}"
@@ -218,8 +272,8 @@ class ExchangeService:
                 f"Trade ID: {existing_trade.public_id}"
             )
 
-        # Validate EUR amount
-        await self.validate_eur_amount(eur_amount)
+        # Validate EUR amount (direction-specific)
+        await self.validate_eur_amount(eur_amount, direction)
 
         # Validate Lightning threshold
         await self.validate_lightning_threshold(bitcoin_transfer_method, eur_amount)
@@ -233,10 +287,11 @@ class ExchangeService:
         # Get and validate price
         cached_price = await self.validate_price_not_stale()
 
-        # Calculate agreed price based on direction
+        # Calculate agreed price based on direction and amount
+        # (includes premium calculation)
         market_price = cached_price.price
-        agreed_price = self.apply_premium_for_direction(
-            market_price, PREMIUM_PERCENTAGE, direction
+        agreed_price, total_premium_percentage = await self.apply_premium_for_direction(
+            market_price, eur_amount, direction
         )
 
         # Calculate sats amount based on agreed price
@@ -263,7 +318,7 @@ class ExchangeService:
             sats_amount=sats_amount,
             market_price_eur=market_price,
             agreed_price_eur=agreed_price,
-            premium_percentage=PREMIUM_PERCENTAGE,
+            premium_percentage=total_premium_percentage,
             status=ExchangeStatus.BOOKED,
         )
 

backend/tests/test_exchange.py

@@ -252,8 +252,10 @@ class TestCreateExchange:
         assert data["eur_amount"] == 10000
         assert data["status"] == "booked"
         assert data["sats_amount"] > 0
-        # For buy, agreed price is market * 1.05
-        assert data["agreed_price_eur"] == pytest.approx(21000.0, rel=0.001)
+        # For buy with €100 (small trade): base 5% + extra 2% = 7%
+        # Agreed price is market * 1.07
+        assert data["agreed_price_eur"] == pytest.approx(21400.0, rel=0.001)
+        assert data["premium_percentage"] == 7
 
     @pytest.mark.asyncio
     async def test_create_exchange_sell_success(
@@ -279,8 +281,10 @@ class TestCreateExchange:
         assert data["direction"] == "sell"
         assert data["bitcoin_transfer_method"] == "lightning"
         assert data["eur_amount"] == 20000
-        # For sell, agreed price is market * 0.95
-        assert data["agreed_price_eur"] == pytest.approx(19000.0, rel=0.001)
+        # For sell with €200 (at threshold): base 5% + extra 2% = 7%
+        # Agreed price is market * 0.93
+        assert data["agreed_price_eur"] == pytest.approx(18600.0, rel=0.001)
+        assert data["premium_percentage"] == 7
 
     @pytest.mark.asyncio
     async def test_cannot_book_same_slot_twice(
@@ -536,6 +540,118 @@ class TestCreateExchange:
         assert response.status_code == 400
         assert "at most" in response.json()["detail"]
 
+    @pytest.mark.asyncio
+    async def test_premium_calculation_small_trade_extra(
+        self, client_factory, regular_user, admin_user
+    ):
+        """Premium includes extra for small trades (amount <= threshold)."""
+        target_date = await setup_availability_and_price(client_factory, admin_user)
+
+        with mock_price_fetcher(20000.0):
+            async with client_factory.create(cookies=regular_user["cookies"]) as client:
+                # Trade below threshold: should get base + extra premium
+                response = await client.post(
+                    "/api/exchange",
+                    json={
+                        "slot_start": f"{target_date}T09:00:00Z",
+                        "direction": "buy",
+                        "bitcoin_transfer_method": "onchain",
+                        "eur_amount": 14000,  # €140, below €200 threshold, multiple of €20
+                    },
+                )
+
+        assert response.status_code == 200
+        data = response.json()
+        # Base 5% + extra 2% = 7% total
+        assert data["premium_percentage"] == 7
+        # Market 20000 * 1.07 = 21400
+        assert data["agreed_price_eur"] == pytest.approx(21400.0, rel=0.001)
+
+    @pytest.mark.asyncio
+    async def test_premium_calculation_large_trade_base_only(
+        self, client_factory, regular_user, admin_user
+    ):
+        """Premium is base only for large trades (amount > threshold)."""
+        target_date = await setup_availability_and_price(client_factory, admin_user)
+
+        with mock_price_fetcher(20000.0):
+            async with client_factory.create(cookies=regular_user["cookies"]) as client:
+                # Trade above threshold: should get base premium only
+                response = await client.post(
+                    "/api/exchange",
+                    json={
+                        "slot_start": f"{target_date}T10:00:00Z",
+                        "direction": "buy",
+                        "bitcoin_transfer_method": "onchain",
+                        "eur_amount": 24000,  # €240, above €200 threshold, multiple of €20
+                    },
+                )
+
+        assert response.status_code == 200
+        data = response.json()
+        # Base 5% only (no extra)
+        assert data["premium_percentage"] == 5
+        # Market 20000 * 1.05 = 21000
+        assert data["agreed_price_eur"] == pytest.approx(21000.0, rel=0.001)
+
+    @pytest.mark.asyncio
+    async def test_premium_calculation_direction_specific(
+        self, client_factory, regular_user, admin_user
+    ):
+        """Premium uses direction-specific base values."""
+        target_date = await setup_availability_and_price(client_factory, admin_user)
+        target_date_2 = await setup_availability_and_price(
+            client_factory, admin_user, target_date=in_days(2)
+        )
+
+        # Update pricing config with different premiums for buy/sell
+        async with client_factory.get_db_session() as db:
+            from repositories.pricing import PricingRepository
+
+            repo = PricingRepository(db)
+            await repo.create_or_update(
+                premium_buy=10,
+                premium_sell=8,
+                small_trade_threshold_eur=20000,
+                small_trade_extra_premium=2,
+                eur_min_buy=10000,
+                eur_max_buy=300000,
+                eur_min_sell=10000,
+                eur_max_sell=300000,
+            )
+
+        with mock_price_fetcher(20000.0):
+            async with client_factory.create(cookies=regular_user["cookies"]) as client:
+                # Buy trade
+                buy_response = await client.post(
+                    "/api/exchange",
+                    json={
+                        "slot_start": f"{target_date}T09:00:00Z",
+                        "direction": "buy",
+                        "bitcoin_transfer_method": "onchain",
+                        "eur_amount": 14000,  # €140, multiple of €20
+                    },
+                )
+                assert buy_response.status_code == 200
+                buy_data = buy_response.json()
+                # Buy: base 10% + extra 2% = 12%
+                assert buy_data["premium_percentage"] == 12
+
+                # Sell trade (different day to avoid one-trade-per-day limit)
+                sell_response = await client.post(
+                    "/api/exchange",
+                    json={
+                        "slot_start": f"{target_date_2}T10:00:00Z",
+                        "direction": "sell",
+                        "bitcoin_transfer_method": "onchain",
+                        "eur_amount": 14000,  # €140, multiple of €20
+                    },
+                )
+                assert sell_response.status_code == 200
+                sell_data = sell_response.json()
+                # Sell: base 8% + extra 2% = 10%
+                assert sell_data["premium_percentage"] == 10
+
     @pytest.mark.asyncio
     async def test_amount_not_multiple_of_increment_rejected(
         self, client_factory, regular_user, admin_user
@@ -595,9 +711,22 @@ class TestCreateExchange:
                 },
             )
 
-        # Create stale price (create_exchange doesn't fetch, just reads from DB)
+        # Create stale price and pricing config
         async with client_factory.get_db_session() as db:
             await create_price_in_db(db, price=20000.0, minutes_ago=10)
+            from repositories.pricing import PricingRepository
+
+            repo = PricingRepository(db)
+            await repo.create_or_update(
+                premium_buy=5,
+                premium_sell=5,
+                small_trade_threshold_eur=20000,
+                small_trade_extra_premium=2,
+                eur_min_buy=10000,
+                eur_max_buy=300000,
+                eur_min_sell=10000,
+                eur_max_sell=300000,
+            )
 
         async with client_factory.create(cookies=regular_user["cookies"]) as client:
             response = await client.post(