User processed an invoice as final, but wants to add some information to it, say a PO number.
You are allowed very limited editing of finalized invoices in Core. You can add or edit the invoice memos, and change the invoice number, provided you have the right security permissions. In this case, you can add the PO number to the final invoice in its memo.
On the Invoices screen, click the row-action menu to select View Memos and add the relevant information to it (preferably in Memo 1 as Memo 2 is disabled for some templates). Check the Core Help for details.