Have you considered doing a forms personalization to prevent this?
Hope this helps
Can we achieve only thru Personalization? Please confirm
As per Oracle Standard Functionality, receiving tolerance in the Item Master / Org Item / Supplier Master / Receiving Options will be just defaulted to PO. Buyer has privilege to override the same.
If you want to restrict buyer to update some of the fields defaulted in PO, you need to do personalization only for meeting the same.
Thanks for the update
As far as I know yes.