How can I add a discount "outside" the invoice

In case anyone’s familiar with it, this is about the 20-cent Discount On Fuel

I have an invoice that has a Total of 51.43€ but we only pay 45.67€.

According to my accountant the invoice I add to Dolibarr should show the full amount 51.43€ but when making the payment 45.67€ go to the regular accounting account and 5.67€ should go to a specific account (740).

I think the TLDR is that total_ht and tva should not be affected by the discount.
https://i.imgur.com/MXaKdgo.png

No one’s ever come across this issue?