If the mistake is something such as an incorrect rate, time, or missing session, just delete the invoice, correct the session data then recreate a new invoice for the practice and month. Remember - if you have already done your pension forms A and B for that month, you will need to delete them and do them again to reflect your changes.
Clicking the PDF button just opens the selected document. To force it to be recreated with the new details, press and hold down the SHIFT key as you click the PDF button.
On the invoices grid, select the invoice you want to recreate, hold down the SHIFT key and click the PDF button at the top of the grid. The invoice is then recreated with the current template and personal details.
PennyPerfect assumes the payment received is always the same as the invoiced amount. It is not a complete accounting package so there is no ability to do adjustments and debts.
You can create multiple invoices for a Practice each month. Your Pension forms are still calculated correctly because they rely on the date and type of the session, not the invoice its on.