Basically for joining a college for pursuing the studies you don require relieving letter from employer.
Moreover you cannot get an experience certificate as well for just one and half months service, which is just of training period.
since you were under the raining period, you may see the clause for resignation/termination during the first six months period which may be termed as training period or probation.
You have to go through the employment offer letter in this regard.
Basic pay is the the one mentioned in the salary slip, if the bond contains that you may to return 50% of basic pay, it would be from the basic pay amount what has been mentioned in the salary slip.
The lopsided agreement by the bank in the employment offer letter shall be valid in the eyes of law, hence you demand the relieving letter after tendring your resignation letter properly. let them give their comments in writing after which you may plan to initiate proper legal action as per law.