Purchase Orders can be locked after final approval which means no amendment possible.

In order to have this option enabled the Admin will need to go to Admin > PO Settings.

Once in PO settings then tick the option 'Lock Purchase Orders after final approval (no amendment possible)' under the Lock P.O Settings option.

 

Once this is enabled on the project after a Purchase Order has been final approved it cannot be amended, the Purchase Order in question will need to be voided however you can use the function ‘Create Purchase Order based of this one’