If you have an enforceable contract, and you are honoring your obligations under it, you must be paid as per the contract--there should not be any legal grounds for paying you less. You could sue to enforce the contract, and recover the difference between what you were paid and what you should have been paid. The first thing to do is to bring the contract to an attorney and discuss with him/her the circumstances under which it was signed and of your employment, to make sure that it is in fact an enforceable contract.