Returns and corrections
Credit notes, cancelling a voucher, and correcting a mistake.
Sale returns
A sale return is a credit note: goods come back into stock at the current average cost, so returning items does not distort the valuation of stock you bought since.
Cancelling
Cancelling a voucher reverses everything it did — stock goes back, and any payment it raised is voided — and marks the document as cancelled. It stops counting towards stock, GST, balances and every report.
Correcting
Editing a saved voucher cancels the original and issues a replacement linked to it. Both documents remain, so the correction is auditable — you can see what was issued and what replaced it.
A voucher cannot be cancelled if the payment recorded against it has since been applied to other invoices. Adjust that payment first; the application will tell you when this is the case rather than quietly unpicking your books.