Thief-Bait looks like a fancy money pouch. It cannot be cut. An evil noble had a few made. He wears them about town in a sort-of obvious location that a pickpocket can easily reach. The noble wanders through an area notorious for thieves and lets the pouch be stolen. He then leaves.
When the thief reaches into the bag, he must make a saving throw vs. Spells. If he fails, the bag shuts itself and cuts off his fingers. It also eats the fingers, so they may not be recovered.
If the thief makes his Save, he manages to get his hand out in time as he realizes the bag is magically trapped.