Don't try and fix this problem with an adjustment. Enter a cash transaction for $0.00 and pick up the credit balance from the overpaid invoice. Then apply the credit to the other invoices to pay them off.

Sorry, but you will never see "unpost batches" in anything other than Adagio Ledger, as the tax and other implications are too onerous to unwind. Just make sure that you make a backup before posting any batches, then you can always restore to that backup.