Credit and debit notes adjust supplier tax liability; credit note timing and restrictions on passing tax incidence apply. If a tax invoice overstates taxable value or tax, suppliers may issue a credit note and must declare it in the return for the month issued but no later than the September following the financial year end or the relevant annual return date; tax liability is adjusted as prescribed and reductions are disallowed where tax incidence has been passed to another. Where a tax invoice understates taxable value or tax, suppliers must issue a debit note (including a supplementary invoice), declare it in the return for the month issued, and adjust tax liability as prescribed.