PO amend below GR value

Dear Experts,

There is a requirement for our team to prevent changing of the PO value after the GR or SES has been done. E.g. If the PO line item value is Rs.1,00,000 and the GR or Service Entry Ssheet has happened for Rs. 20,000, then the po line item value cannot be reduced to any figure of less than Rs.20,000. The check should be at item level, but should allow the user to create new line items in the PO.

Pls suggest.

Regards

Pankaj