Fix tax calculation for negative fees

src/pretix/base/models/tax.py

@@ -206,7 +206,12 @@ class TaxRule(LoggedModel):
                 base_price_is = 'net'
 
         if base_price_is == 'gross':
-            gross = max(Decimal('0.00'), base_price - subtract_from_gross)
+            if base_price >= Decimal('0.00'):
+                # For positive prices, make sure they don't go negative because of bundles
+                gross = max(Decimal('0.00'), base_price - subtract_from_gross)
+            else:
+                # If the price is already negative, we don't really care any more
+                gross = base_price - subtract_from_gross
             net = round_decimal(gross - (gross * (1 - 100 / (100 + rate))),
                                 currency)
         elif base_price_is == 'net':

src/tests/api/test_orders.py

@@ -2128,6 +2128,30 @@ def test_order_create_fee_with_auto_tax(token_client, organizer, event, item, qu
         assert o.total == Decimal('25.30')
 
 
+@pytest.mark.django_db
+def test_order_create_negative_fee_with_auto_tax(token_client, organizer, event, item, quota, question, taxrule):
+    res = copy.deepcopy(ORDER_CREATE_PAYLOAD)
+    res['fees'][0]['_split_taxes_like_products'] = True
+    res['fees'][0]['value'] = '-10.00'
+    res['positions'][0]['item'] = item.pk
+    res['positions'][0]['answers'][0]['question'] = question.pk
+    item.tax_rule = taxrule
+    item.save()
+    resp = token_client.post(
+        '/api/v1/organizers/{}/events/{}/orders/'.format(
+            organizer.slug, event.slug
+        ), format='json', data=res
+    )
+    assert resp.status_code == 201
+    with scopes_disabled():
+        o = Order.objects.get(code=resp.data['code'])
+        fee = o.fees.first()
+        assert fee.value == Decimal('-10.00')
+        assert fee.tax_value == Decimal('-1.60')
+        assert fee.tax_rate == Decimal('19.00')
+        assert o.total == Decimal('13.00')
+
+
 @pytest.mark.django_db
 def test_order_create_tax_rule_wrong_event(token_client, organizer, event, item, quota, question, taxrule2):
     res = copy.deepcopy(ORDER_CREATE_PAYLOAD)