Invert calculation to get sale order amount - invoiced lines

models/sale.py

@@ -19,9 +19,9 @@ class SaleOrder(models.Model):
         for order in self:
             line_to_invoice = [line for line in order.order_line if line.invoice_status == 'to invoice']
 
-            untaxed_amount_to_invoice = 0.0
+            untaxed_amount_to_invoice = self.amount_untaxed 
             for line in line_to_invoice:
-                untaxed_amount_to_invoice += line.qty_to_invoice * line.price_unit
+                untaxed_amount_to_invoice -= line.qty_invoiced * line.price_unit
 
             order.update({
                 'untaxed_amount_to_invoice': untaxed_amount_to_invoice