Service bond is in lieu of securing expenses borne by the recruiter/employer, towards the special training and skill inculcation of the concerned recruit/employee.
If not specialised training etc. was provided to you have proof to substantiate the same, they cannot forfeit the bond amount. In fact when you challenge the bond, the onus will be on them to substantiate that they incurred expenses cumulating to Rs. 7 lacs in extending you a specialised training specially designed for you and the other fresh recruits.
If there is no training, you are not mandated to pay them.