Rejecting an offer from Company A when H1B Petition is approved

I am from India and got a job offer from company A and I have signed the offer letter (Agreement Letter at-will), scanned it and email them. Now the stage is my H1B Petition in approved by USCIS.
But I got a good offer in India itself, and I am willing not to travel outside India and want to reject company A’s offer letter.

Question1: Can I reject their offer?
Question2: Can they sue me or ask me to reimburse the H1B or Attorney fee at this stage where I have H1B Petition approved but have not given the H1B visa interview yet.
Question3: Can they sue me or ask me to reimburse the H1B or Attorney fee at this stage where I have H1B Petition approved and also cleared the H1B visa interview.

In the Company A’s offer letter (Agreement letter at-will) they have mentioned:

“If Employee voluntarily terminates employment within eighteen (18) months of the Effective Date of this Agreement, Employee agrees to reimburse the Company any and all costs incurred by the Company in connection with procuring an Employment Visa on Employee’s behalf and authorizes the Company to deduct the Employment Visa expenses incurred by the Company from unpaid compensation, including salary, accrued vacation, bonuses, and/or unpaid expenses, as applicable, according to all federal and state wage and hour laws.”

I have signed this offer letter in the month of March 2014.

Please help me as this is urgent.