If you want to give a customer a discount on each invoice, set this discount in his relation file:
Define discounts in relation files for customers. Read more about the relation file.
In the Invoice Header Data section you can define the discount % for this customer.
A discount is added to each invoice proposal for this customer automatically.
While you create an invoice, you can add, change or delete a discount.
Discount in % or amount
You can allow a discount as a percentage or as an amount. For amounts, you can choose between an amount including or excluding VAT.
Click Percentage to change it to amount, or vice-versa.