You can try to do a zero dollar adjustment

I was thinking the same BL, but if not Data File Display & Maint is next option.  And you could possible automate with a VI import if applicable.

Generally, posted data cannot be edited.

BL's adjustment might update the comment you see on the Invoices tab of Vendor Maintenance.

ZIP's DFDM could work, but editing raw data is not something that should be considered routine.

If you need to add comments to posted invoices, for reference (or reporting), consider a memo instead comments.