[FIX] test for refund base on amount total not amount untaxed

addons/sale/sale.py

@@ -340,7 +340,7 @@ def action_invoice_create(self, grouped=False, final=False):
             if not invoice.invoice_line_ids:
                 raise UserError(_('There is no invoicable line.'))
             # If invoice is negative, do a refund invoice instead
-            if invoice.amount_untaxed < 0:
+            if invoice.amount_total < 0:
                 invoice.type = 'out_refund'
                 for line in invoice.invoice_line_ids:
                     line.quantity = -line.quantity