All states have their own version of labor laws, what state are we talking about here.  If you leave a employer to join their client, same state have a law against it. If you are just leaving the employer to join any other business, mostly you are safe, despite the term mentioned in the job offer/contract letter. In your case I would consult a lawyer. Situation would be worse if you are on H1, USCIS might withheld your H1 transfer as well. Be on the safe side, get a lawyer.