Exploitation by changing bond rules by college, what action to take?

i have done md medicine and had signed post pg bond by my institute .earlier batches the post pg bond was started once 3 years of residency completes even befoee passing exams , i was also made to join institute before my exams and conpletion of 3 yrs of junior residency as senior resident and was verbally told that this period will be included in bond by after exams results the rule was changed and bond starting datewas considered after exam results. They made us join before exams in name of bond without applying for the post now changing the rule as theywant to extend the bond as institute require staff .