When dealing with a percentage discount for a payment term. Is it possible to always calculate the taxes with the discount awarded on it, even if the customer does not pay soon enough. 

An example of this calculation: 

Suppose you create an invoice for 100 € without VAT:

• the taxable amount (net) is € 100
• the VAT amount on this invoice is 21 €

If you now grant a 3% discount in cash, and your customer pays within 10 days, your invoice must include the following:

• If you pay within 10 days you only have to pay € 117.37:
  • the taxable amount (net) is € 97;
  • the VAT amount on this invoice is 20.37 €.

• If your customer pays after 10 days, they have to pay € 120.37 (and not € 121):
• the amount without VAT is 100 €;
• the VAT amount on this invoice is € 20.37 and not € 21, as the taxable amount is € 97.

So you always enjoy a 3% discount on VAT, even if you have not paid within 10 days.