No clear communication on return of security deposit by the house owner

Hello, We have been staying in a rented flat in Bangalore from the past 1.25 years. The flat is owned by a retired couple with whom we were on good terms until March 2021. Since they ill treated us over a parkong issue, we planned to vacate the house. Considering their age, we did not want to quote anything disrespectful to them, so we just went by the agreement and gave them one month written notice before vacating. Owing to their inability to find new tenants due to the current COVID situation they have been treating us really badly over the past one month. Now when it comes to return of security deposit they have been stating that they would return our amount only after the new tenant comes that too after deducting minimum 20% of the deposit (maximum can go to any amount). No such clause has been mentioned in rental agreement. They aren't ready for a healthy communication and not ready to even consider rental agreement as a written confirmation. It was purely on humanitarian grounds and owing to their age that we did not initially want to create a problem. Please advise if I should send them a legal notice and what would be charges involved.