This situation may be handled in two different ways. You may enter an AP invoice or credit memo to adjust the invoice to the correct balance or you may "void" the incorrect invoice (create and apply an offsetting invoice). If an adjusting invoice is created, the original invoice number should be referenced in the adjusting invoice. For example, if the original invoice was #100, the adjustment might be assigned invoice #100-adj. The invoice number reference will help to ensure that the two portions of the invoice are paid at the same time.