Hi Sunita
Could you check with your ABAPER that after entering PO number and before saving does system save that PO number in some temporary structure like XVBKD may be. I am not sure about it but I hope system must be saving it in some structure and may be ABAPER can validate it. It looks difficult but you can give it a try.
If you cannot control this through ABAP then 2nd possibility could be through authorization check but it depends how many users you have who can create order at the same time and how many order types are there for sale order?
Thank$