Typical C/M CSV transaction

We have been doing a stable third party integration into GP for many years doing AP-AR transactions.

Customer has requested that certain negative A/P transactions be instead entered as Credit memos.

We need to create a credit memo transaction. I am assuming that the general data fields would be identical but we would need the transaction code related to a credit memo. Can someone provide us with this code and/or an example of Hdr/Detail record(s) for a typical credit memo. Thanks for your assistance.