my friend,
first of all bear in mind that here in Qatar employers can do almost everything as the labor law does not help much for employees. they can ruin u and also they can crown u. But if this is a government institute, I think u r somewhat in safe hands. I don't think u can get much legal actions as it will be only a wasting of money and time even u have some positive points such as the breach of original offer by the company. that will not help you much since if your employer has not signed the said contract.

For the rest, better follow the given answers of wannabelikemike, daffodils and oryx as they are well perfect.