Hi Kumar,
How about this....
Your end user is ready to create PO only once right ?
How about you ask the end used to create a PO with a fixed quantity and with the decided net price.
And you on your part configure the system to generate Automatic PO, when the procured quantity is deficit.
Or if the requirement is specifically that all the GRs be done against the same PO, you are going to have to change the PO. I don't see any other option.
Thanks,
Archit