See 18.2 Credits and contingent discounts
When you allow a credit or contingent discount to a customer who can reclaim all the tax on your supply as input tax, you do not have to adjust the original VAT charge provided both you and your customer agree not to do so. Otherwise, you should both adjust the original VAT charge. You should issue a credit note to your customer and keep a copy.
But, you must not issue a credit note if you’re using a VAT MOSS return to make your declarations and payments on business to consumer digital services. This is because corrections can only be made to the original return and not by making an amendment to a later one.
Read How to correct VAT errors and make adjustments or claims (VAT Notice 700/45) for more information on how to correct VAT errors and make adjustments or claims.
