When you fill in a purchase invoice in Isah, you also fill in the VAT sum. A purchase invoice received after the effective date of the new VAT rate but relating to goods delivered before the effective date will still have the old VAT percentage. Because by then the VAT rate has already been set to the new percentage in Isah, the VAT sum suggested during invoice entry will be based on the new VAT rate. You can manually change this VAT sum. The link with your financial application ensures, however, that the correct VAT sum is posted to your financial database.