We do that with our purchase orders to prevent accidental changes after the transaction has been finalized--in our case this is when a script is performed that prints the purchase order and changes a status field in the record to "printed". (we redeem used beverage containers and purchase scrap metal--so these are POS POs where the money travels in the opposite direction than does a POS invoice in a retail environment.)
The privilege set for our users has a lock expression that checks the status of this field to deny editing if the status field = "printed".
See "Editing record access privileges" in FileMaker Help and check out this particular sub section: "Entering a formula for limiting access on a record-by-record basis" for a detailed description of how to set this up.
I will definitely take a look at that.
Thanks for the info.