From 1 September 2019, the VAT accounting regulations will be amended so that, where the price is adjusted after the supply has taken place, suppliers and customers will have more certainty. Where the price is increased, the increase will be deemed to take place when both parties to the transaction have agreed to the increase. The supplier will then have 14 days to issue a debit note.
Where the price is reduced, the price reduction will be deemed to take place when the suppliers makes a refund to his customer. In both cases, the supplier will adjust the VAT payable or repayable in the VAT period when the increase or decrease occurs.
Source Grant Thornton