You can modify the invoice if no payments are received for that invoice.
Only the school admin and the teachers have the privilege to modify the invoice
To edit the existing invoice, go to the Reports page under the Accounting menu.
You can select the Edit Invoice option from the drop-down menu under the Status column of the invoice.
By selecting the Edit Invoice option, you will enter the edit invoice page, shown below.
Here, you can modify each item and add/remove credits for the existing one in the invoice.