For the "bill" itself, we can't see the other side, but there's enough differences that you wouldn't mistake it for real currency at more than a glance. Fake bills for movie/tv sets look far closer to real bills (basically with obviously fake serial numbers and "for movie use only" stamped on it.

For leaving it as a tip, it's not being used to satisfy a payment. While tipping is expected, it's not required and there's not an agreed upon amount between the parties. So they're just leaving a scummy flyer behind. Not any different than the people that 'tip' with Chick Tracts. If they were to use it to try and pay the bill, it'd be illegal.