diff -r 198974b41315 sale.py
--- a/trytond/trytond/modules/sale/sale.py	Tue Jun 21 15:24:23 2016 +0200
+++ b/trytond/trytond/modules/sale/sale.py	Tue Jun 21 15:25:27 2016 +0200
@@ -1366,11 +1366,17 @@
 
         skip_ids = set(l.id for i in self.sale.invoices_recreated
             for l in i.lines)
+        is_in_invoice = Transaction().context.get('in_invoice', False)
         for old_invoice_line in self.invoice_lines:
             if old_invoice_line.type != 'line':
                 continue
+            if is_in_invoice and 'out_' in old_invoice_line.invoice_type:
+                continue
+            elif not is_in_invoice and 'in_' in old_invoice_line.invoice_type:
+                continue
             if old_invoice_line.id not in skip_ids:
-                sign = (1.0 if invoice_type == old_invoice_line.invoice_type
+                sign = (1.0
+                    if invoice_type[3:] in old_invoice_line.invoice_type
                     else -1.0)
                 quantity -= Uom.compute_qty(old_invoice_line.unit,
                     sign * old_invoice_line.quantity, self.unit)