Use "cancel" method instead of "refund" for free orders (#743)

* Use "cancel" method instead of "refund" for free orders

* Adjust API

src/pretix/base/models/orders.py

@@ -213,6 +213,12 @@ class Order(LoggedModel):
     def net_total(self):
         return self.total - self.tax_total
 
+    def cancel_allowed(self):
+        return (
+            self.status == Order.STATUS_PENDING
+            or (self.status == Order.STATUS_PAID and self.total == Decimal('0.00'))
+        )
+
     @staticmethod
     def normalize_code(code):
         tr = str.maketrans({
@@ -272,7 +278,7 @@ class Order(LoggedModel):
         """
         positions = self.positions.all().select_related('item')
         cancelable = all([op.item.allow_cancel for op in positions])
-        return self.event.settings.cancel_allow_user and cancelable
+        return self.cancel_allowed() and self.event.settings.cancel_allow_user and cancelable
 
     @property
     def is_expired_by_time(self):

src/pretix/base/services/orders.py

@@ -245,7 +245,7 @@ def _cancel_order(order, user=None, send_mail: bool=True, api_token=None):
     if isinstance(api_token, int):
         api_token = TeamAPIToken.objects.get(pk=api_token)
     with order.event.lock():
-        if order.status != Order.STATUS_PENDING:
+        if not order.cancel_allowed():
             raise OrderError(_('You cannot cancel this order.'))
         order.status = Order.STATUS_CANCELED
         order.save()