Processing vendor credit notes on existing purchase order

My team has received vendor credit notes which we want to process on the vendors current purchase order. The team within my company that administrates MS Dynamics advised a negative value line and a positive value line needs to be created to post the credit note whilst offsetting the reduction in PO value.
Our subcontracts team that is the main procurement user have taken issue with this process and we are currently in a situation where we have no clear guideline on how to process vendor credits.
My company works on SAP in the US so think of the PO process in that manner. Totally understand MS Dynamics AX is different and wanted to know the full options.