How to change sign on amounts fields in the credit memo received from OBN?

We are receiving credit memo from suppliers via OBN. All other suppliers send negative invoice amount on credit memo and standard XSL is checking sign of invoice amount to determine standard invoice vs credit memo.

We are implementing a new supplier and this supplier is sending the credit memo wot all amounts with same sign as in original invoice and a purpose tag to indicate credit memo. We were able to update the purpose tag to change the invoice type to CREDIT, however we also need to reverse all amounts sign. All positive invoice header and line amounts needs to be changed to negative. We also have some discounts that were with negative sign, we need to change those to positive sign.