Legal action against hostel owner not returning deposit money

I stayed in a PG hostel in Chennai. I gave them an advance of 6500 and I gave more than 1 month notice period and vacated the hostel. While vacating, hostel manager promised me of returning (Advance - Electricity bill). Later after 1 month 1 week time frame they are saying there is a clause that min 3 months stay is required which was not communicated to me while joining. One week after joining PG, they made me to sign a rental agreement and I clearly cleared out the min 3 month stay clause and taking advance also clearly mentioned in the agreement. I don't have the agreement but they definitely have it. And I have all mail communication, where they agreed and talked about advance and I have my roommate and couple of PG guys as witnesses. How can I take things legally. I am moving out of India next month. Should I be available in person. Its not about how much money involved in this case, its more about how these people are thinking themselves above law.