Landlord not returing the security deposit of 50000

I was renting a flat for 4 yrs but now as the flat was too old and owner not ready to repair the interior and also he was increasing 10 percentage on every yrs renewable I thought to vacate the flat, the leave and licence agreement was expired on dec 8th and in jan 17 I called him over phone but he did not responded. The owner is in USA and he has a POA given to his friend and called both them but they did not picked my call on 17 of jan so I whatapp them saying that the flat would be vacated on or before 7th feb but I vacated the flat on 31jan itself and send him an email that the flat is vacated and return the deposit and take over the keys. but now they have return back to me saying that they will deduct 2 month rent as I have not given them written notice and they say whatapp msg is not legal. the I had informed him to deduct the jan rent from the deposit and return back the balance amount but they are just over msg reply and stick to deduction of 2 month rent and they will recover