Trespasser in flat

I have given my flat in Kolkata to my friend at the time of his marriage with a condition to vacate the flat in sometime after marriage. Its being 2.5 years now he is not vacating the flat and asking for more time. he is asking that he want to buy the flat once his bank loan is approved. I have not done any legal agreement like leave and licence agreement or anything else with him. As a token, he is giving Rs. 15,000/- per month as rent in my sister's (married) bank account or sometimes in cash but there is no entry in my account as such. Since we know each other for many years he earlier gave my loan of Rs. 5,00,000 (before my flat was construction completed). The loan amount is still with me. Now i am just asking him to vacate the flat. i am ready to repay him his 5 lakh rupees. The flat papers are with bank as housing loan is taken against that flat. Please advise what i can do next to get me flat back legally. I reside in Mumbai and only my sister is in Kolkata now.