Is there anyone who's familiar with commission/royalty program that
pays the third party for introductions that lead to new customers.

Let's say we have a customer who's willing to refer new clients to us,
and we would like to pay x % of the new sales to this customer as
commission/royalty.

But how does this customer know that he is getting full commission? As
he doesn't know the new sales amount. Do we have to show the invoices
to him?