Invoice field validation and discount rates

In the UK or Belgium, you can give a discount rate for early payment of an invoice if you selected the "Allow tax calculation on reduced net amounts" option. Because of this, the normal Invoice Validation for VAT does not apply, and the tax amount is calculated using the following equation:

(NetAmount1 - Discount) * TaxRate1 = TaxAmount1, where Discount = NetAmount1 * DiscountRate

The document does not usually include the reduced net amount, just the discount or the discount rate.