Mr. Jamadar,
I can understand under the circumstances that you must have signed the bond. Off course you cannot say you were forced to sign it. On the other hand you need not take that defence also. The bond that you have signed needs to be approved by a court of law under the Contract Act also. That apart unfair labour / employee practice is also an issue here. We have delt with such situation several times for our clients and have shown the employer where they stand in the eyes of law. They cannot think, just because they are a company and you an individual will not be able to take their might and cannot fight them if challenged, and meekly give up Rs. 50,000, I don't think so. I would first retaliate with a harassment (legal) notice against them, first and then wait for their reply. Its only then we know how serious they are about extracting that money from you, even a average lawyer will make them spend at least half of that amount to begin with. If you need any help in Bangalore do let me know. Do not fret, you have not committed a crime, the bond is a litigation-able if need be, and you will succeed for sure.