Landlord not refunding security deposit

I have taken a flat on rent in June 2018 and an rental agreement signed of 11 months. After 11 montb we have not renewed the agreement and keep on staying with mutual understanding with incremental rental of 5%. However in Oct 20, I was getting same type of flat at nearby locations at 40% minimum rent heb e I negotiated with my owner and he agreed to cut my rent by 25%, ( No written evidence) . However now on 25th September 2021, my owner asked me to deposit rent with 40% incremental rental saying covid time is gone now have to pay rent as usual, I refused tonpay hence he asked me to vacate the property. I vacated in within 2 days . However I have paid 4 month of maintenance in advance in September for the period of Sept to Dec 21 Now my owner wants to deduct rental of 2.5 month saying I have not given any prior notice, and refusing to pay back complete amount. Our rent agreement is already expired 2 years back hence none of T&C applicable now since agreement become void. I want to send a legal notice to my owner to pay back my refund. Please let me know the process for the same.