How to limit changes to POs created from Requisition

Hi -

In our business process, PO's created from requisitions (via iProcurement) do not need to go through approval process since the approval process is completed on the purchase requisition.  Thus the buyer can self-approve PO's created from requisitions.

However, this configuration allows the buyer to add lines or make changes to the PO without submitting for re-approval.  Ideally, any changes should be done through a purchase requisition, which would be submitted for approval and then the changes are transferred to the PO.

Is there a method to prevent the buyer from directly making changes to a PO created from a requisition and then self-approving the changes?  We still need to allow the buyer to self-approve the PO during the initial creation from the requisition.