Hi,
AT first you have to create "Commission Agent " in the system as one of the Sales representative
assign him to customer in XD02
You can very well use Rebate agreement for sales representative ( who can be a commission agent)
and while processing based on the sales representative Commission rebate is determined
After the Invoice, you can accrue and settle the rebate
only catch here is to create commission agent as sales represent ( either internal or external number range)
hope it is useful