It is possible to allow payments by discount coupons.

First go to Settings > General > Organisation and enable the coupon payment for the organisation.

If the organisation has multiple departments set the department specific coupon use. Go to Settings > General > Department > Coupon to set the department specific use.
You can inherit the settings from the organisation level settings or enable/disable separately.
It is also possible to allow adding coupons for draft invoices.

Then you are ready to pay invoice with a coupon.

Finalizing invoice, to use coupon for the payment choose Not payed yet option.
You will see the payments area with the coupon payment option (here named as Discount Voucher).
Add the amount paid with the coupon and the due sum will be updated accordingly.
Pay the rest of the invoice as usual:  Select +Payment and add the Paid sum and Payment type. Save.

The coupon payment is reported in the end of day report as defined in the organisation settings.