First of all, while it is possible to sue someone for almost anything, the reality what is more likely is the likelyhood that this will be sucessful.
First of all, the difference between what you were allegedly offered, and what you are being paid is relatively small ($7,000) and legal fees would likely eat up much of your award if you were to be sucessful.
Second: You apparently DID sign a contract that you said was "post dated" to the hire date with $52,000 with benefits. This contract will be used in evidence, with the claim that you did sign it at the hire date. (You of course will claim that it was signed later under duress)
Obviously, any suit will be a source of friction between you and your employer, and it is unlikely that you would progress (or possibly have your contract extended) at the end of your current contract.
You need to have this case, and the contract reviewed by an attorney in your area, considering of coures the potential benefits and risks of any such approach.
Other options - negotiate a raise (if your work is worth of a higher wage
, and the employer is otherwise impress with your work) or find another job, this time get the offer in writing (contract) BEFORE you accept the position.