Credit and debit notes: mechanism for adjusting GST liability when invoices overstate or understate taxable value. If a tax invoice overstates value or tax, or goods are returned or deficient, the registered supplier may issue a credit note with prescribed particulars and must declare it in the return for the month issued or by the September following the financial year end or with the annual return, with prescribed adjustment to tax liability; reduction is barred where tax incidence was passed to another. If a tax invoice understates value or tax, the supplier must issue a debit note (including a supplementary invoice), declare it in the monthly return, and adjust tax liability as prescribed.