I have a question.. if someone marked a purchaser order pending billing.. but nothing has been received and no items are listed in related records.. can they not just change a quantity and something like special instructions or memo field to change the status of the purchase order back to pending receipt, under normal circumstances ?

Technically user roles with full permission to PO should be able to edit the PO with the status pending billing or receipt unless it is locked by the approval workflow. If it is locked by the workflow then you may need to add exceptions for the specific user roles in lock record workflow action.

thank you.. not sure how the users accomplish these things..

by any chance you are using approval workflows for the PO?

i am checking now but it doesn't appear so.. this PO has been changed so many times in different areas. I suggested they just zero out all the items and just reissue the PO. The same user has changed the status of this po a few times by adding new line items/quantities