Thank you Moazzam. We have written the logic in the user exit and tested in the sandbox. It is working fine. It is automatically overwrites the PR00 ( KOMV-KBETR) price in each line item of the Credit memo request. and successfully created the credit memo.
In the user exit we have kept the logic to trigger only if this particular document type (ZRTC) is created.
We didn't keep in Alt.calculation type.