What I remember from the spiffs that my car-dealer clients received was that they were not directly tied to sales.  It was more a reward for not pointing customers to other suppliers.  The per-transaction payment bothers me here, but the underlying fact is the same:  a person can't have two masters, and if you have an employer, you can't harm the relationship with his customers by pointing them in the wrong direction.  If the supplier who appreciates the referrals (that help your employer make sales) wants to give monetary thanks, who are you to turn it down?  But you were just doing your job.