Restrict buyer from editing the PO lines during "Process Requisition"

Summary:

Can Oracle restrict buyer from editing the PO lines during "Process Requisition". The problem we are facing is that non-catalog requisition raised for $1000 and requisition approved. Now when buyer goes to "Process Requisition" screen he is able to change the line qty and price. PO created for $2000 from the "Process Requisition" screen. This is causing the PO amount to be different from the linked requisition amount, and buyer is able to do so without any approval.

We do not want the buyer to be able to edit the lines in the "Process Requisition" screen. Please confirm if that is possible