Allow negative currency values for "extra" line items (#3311)

InvenTree/InvenTree/fields.py

@@ -63,10 +63,15 @@ class InvenTreeModelMoneyField(ModelMoneyField):
         # Set a minimum value validator
         validators = kwargs.get('validators', [])
 
+        allow_negative = kwargs.pop('allow_negative', False)
+
+        # If no validators are provided, add some "standard" ones
         if len(validators) == 0:
-            validators.append(
-                MinMoneyValidator(0),
-            )
+
+            if not allow_negative:
+                validators.append(
+                    MinMoneyValidator(0),
+                )
 
         kwargs['validators'] = validators
 

InvenTree/order/models.py

@@ -957,6 +957,7 @@ class OrderExtraLine(OrderLineItem):
         max_digits=19,
         decimal_places=4,
         null=True, blank=True,
+        allow_negative=True,
         verbose_name=_('Price'),
         help_text=_('Unit price'),
     )