Fraud by private hostel management

I am a college passout student who is doing his internship in a company for 3 month and was in need of a place to stay for this period. I took shelter in a private hostel where I was made to sign a form and fill only the basic details by the hostel owner (I did not write any date of leaving there). He charged me ₹8000 as caution money(refundable and mentioned in the bill) and ₹7000 as rent amount. After 3 months approx, now I plan to move out of the hostel to, but the owner refuses to return the caution money and apart from that harasses me daily to pay the fees for the current month. I had not written the date of leaving on the form but it was later filled by himself as per his wish. Now he wants me to pay for the next month or leave the hostel without refunding the caution money (As the form states a date later than the period in which I am leaving). I am tortured daily for this and am planning to seek help. Please guide me what should I do and how am I protected under the law by such frauds.