Atif,
It appears that the PO is retaining the value that is derived and the new value in the PR is not being copied into the PO. Once the PO line item is created with reference to a requisition, the copy function is no longer invoked. So changes to the requisition will not flow into the PO. You can create a reassignment rule for the PO to correct the PO using FMCT. Alternatively you have to delete the PO line item and recreate it with reference to the requisition line item.
Thanks
Shyam