Lock-in period in rent agreement renewal

My owner put lock-in period of 6 months in agreement while renewing it after 11 months. We didn't check the agreement carefully and signed it. Now we informed owner about leaving the flat one month in advance (7th Nov). Owner agreed to it and put post at various places for tenants. Multiple people came to see the flat in one month. We vacated the flat after one month(30th Nov). After that owner started avoiding our call and chat. He kept giving different excuses for delay in returning the deposit. After 20 days he returned 60% of deposit(25K). After 2 more days he denied returning the remaining amount(15K). Now he is saying that since we have not given the notice in written, he will not return the remaining amount. He has rented his flat from 15th Dec. Lock-in period completed on 30th December. Still he is holding 15K which is more than one month rent. Can I get legal help in this case? Can owner hold the money even after renting his flat to another tenant?