My last employer has refused to pay me joining bonus

Hi Experts,

This Girish from Hyderabad. I really need a favorer from you. I do not know what to proceed.

I joined one multi dollar semiconductor company in July 2010. In offer letter, they had mentioned that, if i leave company withing one year then, I need to payback joining amount / bonus 50 K back. But, After three months, one Big company acquired it and new company main office for India in Bangalore rolled out new assignment letter and new offer letter. In new offer letter and assignment letter they have not mentioned anything about joining bonus terms. New offer letter talked about all terms like designation, compensation and other facility. I left this company in 8 months. in final settlement they deducted the joining amount from last paycheck.
in fact, I served notice period according new company policy like one month not 2 month according old company. Got relieving letter with new designation on old company letterhead because New company's Indian office is acquiring this old company's noida office so, it has been taking little bit long.

as written in new assignment letter:
This assignment letter issued to you
will supersede and replace any prior verbal or written agreement between you and xxx company relating to your employment or any aspect thereof any and all prior employment agreement with exception of agreement related to proprietary information"

Please, let me know, what should i do ? I would highly appreciate your valuable reply.

Thank You.


Warm Regards,
Girish