Hi,
I am not sure if this can work, but what is the implication if I think in lines of creating a discount for customer and assign it to master data ? in that case APP should be able to take care of it. I tried manually by creating a invoice through FB70 and putting in a discount amount and got the following accounting entries at the time of incoming payment. Pls check if it works, you can assign the commission GL here.
best regds
Subha