H1B - Is serving Notice Period Mandatory (Very Urgent)

[funnybone]

All, I am in a serious problem & would need assistance from senior members on this forum.

I have transferred by H1B to a new employer & triggered my resignation process with my existing employer. However, my existing employer is not releasing me by 15 days citing project reasons (loss of revenue, escalations etc etc).

As per my offer letter, I am supposed to pay USD "X" in lieu of serving 60 days notice period. I am ready to pay USD "X", but my existing employer is not releasing me with 15 days notice. There is no other clause or any other penalty or liquidated damages mentioned on my offer letter.

1. Is it mandatory for me to serve entire 60 days notice as claimed by my existing employer ?

2. As per my offer letter, is it legal for me to quit by serving 15 day notice period ?

3. Would I sued by my exisiting employer if I just quit after serving 15 day notice (I might not get a proper relieving letter, but I am even fine with that if there is no other way out for me) ?

Request senior members / other members to provide their comments / suggestions as I have to join my new employer quickly or else the employment offer would not be valid.