Schrödinger’s carrot : a motivation that, depending on the final circumstances, might turn out to be either a reward or a punishment
This word is my coinage.
A friend was taking a course in Catalan from a DC nonprofit, the Global Language Network, which offers instruction in a wide array of tongues. The classes are free, but a student must make a $150 deposit at the start of a course. If the student attends all of the classes, they get the deposit back. If they miss classes, they forfeit the $150.
Neither definitively a carrot, nor exactly a stick: Schrödinger’s carrot.