This what is called in french “ristourne” (I do not think there are different terms).
Normally it should be booked in a specific account (in Belgium: 608 and 708, in France: 609 and 709).
The proper tax must be set depending on what kind of product/service is rebated. It seems also that it is possible to skip the taxes.
So I do not think a tryton product should be filled on the invoice line because this is a purely financial operation.
But now I’m wondering if it would be good to have a way to apply the rebate to the unit price of the corresponding stock moves like it is done with landed cost. This way the margin reports are more accurate.