Receiving part in RMA

Hi, I have a part which was not tracked by serial at the time of issue, later it got tracked by serial in the system. Now i m trying to return the part which i issued without serial. system is raising error that system requires unique serial for each line.

when i put serial system raise error saying this serial is not registered in part catalog. 

how can i receive this serial tracked part without registering the serial at the time of RMA receipt, and then to identify its serial from inventory part in stock.

Thanks