There was a change to the VAT legislation recently with regards to how a supplier must account for VAT on invoices where there is a contingent discount, e.g. such as if an invoice is settled with a certain period.
Previously, if a supplier was offering a contingent discount they could calculate the VAT on the invoice on the assumption that the discount was taken even if it was not. HMRC felt that they were potentially losing out on VAT and therefore have changed the rules.
Now a supplier must account for VAT based on the full amount before any contingent discount is applied and subsequently adjust for any discount taken only when the discounted payment is received. The supplier should put a note on the bottom of their invoice to say that if the discount is taken then the revised net, VAT and gross amounts will be x, y and z. If this note is not included they will need to issue a credit note to the customer for the amount of the discount.