Credit Memo Invoice referenced to a invoice

Hello Dear Parthners,

I have a requirement to create a Credit Memo referenced to a invoice for the purpose to see that reference into the transaction FBL5N field Asigment Number (structure RFPOSXEXT- field ZUONR). I think the problem is because when create the Credit Memo Request we delete the materials copied from the invoice and we add a specific material for a credit memos (this credit memo its like a financial credit note).

We want to do this  just for have the link between the credit note v.s. invoices.

Attached you cand fund my configuractions

Thakyou, Regards!