Fix AttributeError usage when creating external refund matching pending sum

src/pretix/base/models/orders.py

@@ -1698,7 +1698,7 @@ class OrderPayment(models.Model):
         })
 
         if self.order.pending_sum + r.amount == Decimal('0.00'):
-            self.refund.done()
+            r.done()
 
         return r