1. Did you inform your landlord in Feb 2015 orally or through a written note. In case you did not inform your landlord through writing in Feb 2015 about your vacating the Flat in 3 months' time, atleast inform the landlord in a written form now, so that you will have some evidence for having informed him in advance.
2.,Since the rental agreement is not a registered one, you can not approach the court to seek legal remedy.
3. Try to get a tenant, on behalf of the landlord, for the Flat as soon as possible, so that you can vacate the house after getting your advance amount.
4. Since you are not keen to renew the rental agreement for a longer period, both of you can extend the rental agreement for a few months only till the landlord gets a new tenant and the same may be endorsed in writing on the old rental agreement.