Hi Naga,
I think you are talking about the clearing lock out here.
In SAP, if there are open Credit items sitting in the account, then they will get clear during the account maintenance run during invoicing or during the auto clearing job which normally runs overnight.
If you want to issue a customer refund, then mark the item for refund.
If you are planning to issue the refund immediately, then you can change the due date of the credit item to current date and run the FPY1 for generating the refund entries. Make sure that you run the auto-clearing or the invoicing run after you have completed the refund run.
However, if you are planning to issue the refund after a certain period of time, then you have to put a clearing lock against the work-item, so that it doesn't get picked in the auto-clearing or the invoicing run. On the day of generating the refund, you need to removed the lock other wise it will not get picked up in the refund run.
Hope it helps...
Thanks,
Amlan