Encountering errors while importing credit memos from third party

We are constantly facing unexpected behaviours where in some credit memos are successfull but some are erroring out with different amounts and quantity errors.

Scenario 1: There is an exisiting unpaid credit memo against PO ABC. I was successfully able to create one more credit memo for the same PO same line without any errors.

Scenario 2: There is an exisiting paid credit memo against PO PQR. Oracle gave me below errors to create the credit memos.

There have been certain situations in which our business is using a negative quantity or negative amount and it does allow sometimes.

Can you please provide a doc or confirm the functionality on how credit memos worked with billed quantity and amounts? what is the ideal process to process credit memos??