Hi Anil,
Can you share the invoice document because
If invoice is posting more than the po amount while creating invoice for difference amount you have given the gl or OBYC - DIF system has taken?
If GL is giving manually you can check the invoice document what is the difference gl, the same gl you can use while reversing the document or while passing the credit memo for difference amount.
I hope it will help to resolve the issue.
Regards,
Nivas.