Also check your offer letter-does it mention that management has mentioned that rules,procedures etc are liable to change at discretion of management.
Normally bonus is recovered if employee leaves within 12 months of joining.
However you can fight it out, if you have the time and energy since there are some looopholes like no mention about repayment etc in your case.
This will have tax implications also.
21st April 2015 From India, Pune