Validity of Agreement with the Employeer

Hi,

I am a citizen of India and currently residing in USA from past 5 years.

The question is my earlier company, which is an Indian company sent me on-site to USA on Feb-03. For this on-site assignment I have signed an Agreement for a period of 12 months stating from Feb-03. Even though the initial On-site assignment was for duration of 12 months but the assignment extended up to 36 months.

After 36 months I have changed my company. Now my previous company asking for monitory penalty for breaching the contract.

I would like to know is this agreement enforceable as the agreement was only for 12 months and the company forced me to stay on-site for 36 months. There were no any additional agreement has been signed as extension of the initial agreement.

Your opinion on this will be a great help.

Thanks