When looking up open entries during Verifikation entry (either from the Verifikation line or the Cross-references table), the system deducts any cash discount from the amount suggested in the Verifikation line if the payment qualifies for a cash discount.
This means that the Regdat (if necessary overridden in the Förfdatum field) and any previous payments of the invoice took place on or before the Kontant förf.datum, and that the remaining amount is zero after deduction of the cash discount.