Can I add a discount after the net invoice amount?

XML and business rules
1

I have an invoice as follows:
Total = 30.90
Discount = 10
Tax = 3.14
Net = 24.04
I want to discount 0.04 from the net invoice amount so that the net is only 24
Where do I pass that discount in the xml file?
can use cbc:PayableRoundingAmount ?

2

After Tax …No

but as a mean of payment, you can create something like cash, mada, … , you can call it “fraction” for example for these small amounts