Extra Rent Amount Is Being Taken By Flat Owner

Hi Sir, I live in Bangalore in a rented flat, we started living here in 2015, we paid 1 lacs advance+1 month rent in advance, After few years the orginal owner had sold that flat to different person and from that month new owner started collecting the rent from us. I paid the last rent in the Month of September and left the flat on 1st of October but flat owner is also deducting rent amount from security deposit. Since we were paying rent in advance he should not deduct rent from security deposit but he is not ready to return the deposit back, What should i do now?