Quite a silly mistake, really.

auction/models.py

@@ -17,10 +17,10 @@ class Inquiry(models.Model):
 
     @property
     def is_paid(self):
-        if not inquiry.invoice:
+        if not self.invoice:
             return False
 
-        return inquiry.invoice.is_paid
+        return self.invoice.is_paid
 
     def get_hotels(self):
         pass