If you have no reason to leave early, then you should expect to forfeit you earnings as stated in the contract. It costs money in lost production for an owner to find a driver, then find another replacement driver.
If you have a valid reason for early exit, you might speak with the owner first to remedy the situation.
I can't imagine the cost of an attorney would be worth an opinion. If the opinion is owner can withhold your money, you are out your money plus costs of opinion. If attorney says owner can't do it, you can go to court and the cost of suing for your "improperly" withheld earnings would be way more than what you might lose. Then you'd still have to find a way to collect the money you are owed. I'm guessing the contract doesn't allow you to recover attorney fees and court costs if you win, but might provide that you have to pay owners fees and expenses if you lose.
I'll have to agree with those who said it is a learning experience. Work out your objections with the owner, or just stick it out until the time constraint has passed, then leave.