Supplementary invoices for price revisions permit outward supply adjustment, subject to recipient reducing corresponding input tax credit. For contracts made before the appointed day, suppliers may issue supplementary invoices or debit notes within thirty days for upward price revisions, and supplementary invoices or credit notes within thirty days for downward price revisions; such instruments are deemed to relate to outward supplies under the Act, and reduction of the supplier's tax liability on account of credit notes is permitted only if the recipient reduces the corresponding input tax credit.