How do I enter a cash discount?

I have an invoice for an investment on which a 2% discount was granted if I paid within 8 days. The vAT is calculated at EUR 7,938, the MvH on which the conditional discount is based. However, the total invoice amount is EUR 9,766.98, or 7,938 + 1,666.98 vAT + 162 discount. Dexxter does not accept this and says that my total amount must be a maximum of EUR 9,604.98, i.e. without the discount. How do I resolve this? Thank you.