US firm failed on offer

I am an Indian student doing Masters in India and a US company has offered me job, through campus recruitment procedure, for their Indian office. I agreed for the salary offered and signed the contract . Since I signed the contract, I was not allowed to take part in rest of the recruitment process. Now I am about to finish my course and join the company, they sent me a letter telling that the salary has been reduced. By getting my signature at the begining of the placement process, I lost other opportunbities and now I am stuck with these people. Is this legal for a company with its parent company in the USA?