Supplementary invoices permit post-contract price revisions and tax adjustments, conditional on recipient reducing input tax credit. Where a pre-existing contract leads to an upward price revision on or after the appointed day, the supplier may issue a supplementary invoice or debit note within thirty days, which is deemed to relate to an outward supply. Where price is revised downward, the supplier may issue a supplementary invoice or credit note within thirty days, deemed to relate to an outward supply, and reduction of the supplier's tax liability is permitted only if the recipient correspondingly reduces input tax credit.