Negative lines on a PO

Hello,

We are using POs to encumber funds against Grants (project_id) within Payables. If AP makes a mistake and keys an invoice which should have included a PO number but did not, the encumbrances are then overstated (i.e., funds are committed to the PO that have already been spent due to the PO number not being captured in AP).

Currently our users in Purchasing then add a negative line to the existing PO and set the amount equal to the value of the invoice. Is there a better way to do this?
Below is an example explaining why they add the negative PO line. However I am wondering whether something else could be done, see second screenshot.