If an invoice has been sent and the invoice amount needs to be changed later on, then this can still be adjusted. This is only possible if there is still an amount outstanding. A fully paid or credited invoice can no longer be adjusted.To adjust the invoice amount, look up the relevant invoice.At the top of the invoice you will see a number of light blue buttons. One is "Add (credit) invoice line". Here you can lower or increase the amount. In the case of a reduction, make sure that a minus symbol is placed in front of the amount. An extra invoice line will now be created.If a payment is being processed, the invoice cannot be adjusted at that time.