Protagoras takes on Euathlus as a student of law under the understanding that his first win in court will be the fee to cover the teaching. But Euathlus decides not to be a lawyer and not to pay, and is taken to court by Protagoras.
Protagoras: “Should you lose the trial, I will get my money, and should you win you owe me the money as we agreed!”
Euathlus: “Should I win the trial, the court has found our contract invalid so I won’t pay. Should I lose the trial, I won’t have won a trial so by our initial agreement I should not pay.”