KB72
Once you have applied a Receipt to a Sales invoice, you are no longer able to change the value of that invoice (because the value of the invoice and its allocation is stored in the receipt data).
You should never change an Invoice that has already been issued. If you have already issues the Invoice, please see How to fix an invoice that has been issued for goods at the wrong price for more options.
If you need to change the value of a sales invoice, the following process can be used: