It's called a "kickback" and in most areas they are illegal.

If you were ever to get into any sort of .gov building (school, hospital, university, etc..) and you received or gave any sort of kickbacks you could face jail time.

As mentioned above gift cards, vacations, etc.. They are all considered kickbacks.

I once bought a cup of coffee for a guy at a major university, a lousy $2 cup of joe, well someone higher up saw that and called my manager saying that if it happened again we would both be tossed off campus and the university employee I bought the coffee for would be terminated. I told my manager the guy forgot his wallet and he borrowed $2, he knew the gig and kind of smirked at me.

No joke, that is how serious some people take this stuff.