How to prevent buyer from manually changing the quantity or amount on PO with a backing requisition

Hi All,

We have the following scenario:

Requisition created for quantity = 2 and amount = 50. The requisition gets approved.

Buyer while converting into a PO (process requisition lines) manually changes the quantity to 5,amount = 10, charge account and creates a PO and PO gets auto approved .

In this case the approval was obtained for lesser qty.

Is there a way to prevent this ? whether approval can be triggered when there is a change from PR to PO.

Also in this case the requester is the buyer.

All inputs are appreciated.

Regards,

Sruthi