Credit note issuance and adjustment permit correction of excess tax or returns, subject to passthrough prohibition. If a tax invoice overstates taxable value or tax, or goods are returned or deficient, the supplier may issue a credit note and must declare it in the return for the month of issue but not later than the September following the financial year or the annual return filing, adjusting tax liability as prescribed; reduction is barred if tax incidence and interest were passed to another person. If a tax invoice understates taxable value or tax, the supplier must issue a debit note (including a supplementary invoice), declare it in the return for the month of issue, and adjust tax liability as prescribed.