Hi Pratik,
If you are sure that there is SP assigned then follow this steps. From AR Invoice choose Copy to button and select AR Credit Memo. On AR Credit Memo select the row 2 (Complete Select) and right mouse click and delete that row. After deleting row add the document.
If the document added succesfully that mean there is an validation through SP. and if it is not I suggest you to rise a ticket to SAP.
Thanks,
Harshal