How to setup sign positive/negative, in INVOICE TYPE for a DEBIT MEMO or CREDIT MEMO on AP module

Hi

I need setup the sign positive/negative for a DEBIT MEMO and CREDIT MEMO, the issue is that in a purchase order was made a return to supplier, so in automatic an invoice is created with description: "System Generated by 'Return to Supplier' Transaction".

The problems is that is invoice is created like DEBIT MEMO but with NEGATIVE SIGN.

Regards.