Once an invoice is posted, you cannot modify any of the charges and payments that are included in the invoice. If you need to modify an invoice once it has been posted, you can unpost it by selecting Billing:Invoice:Unpost:
The invoices that can be unposted are shown:
You can select any number of invoices.
The following commands are available for processing:
Note that you can only unpost invoices that do not have any payments posted against them, when the post payments against invoice is turned on.