Developer not returning invested amount on cancellation requesst.

Hi There, I had booked the flat with Marathon Realty (Marathon Nexzone Panvel) in Jun 2016 in under construction project at Panvel with legal registration of flat, and till Nov 2019 I paid almost 50% of requested amount. The possession of flat was agreed on Sept 2020 however that was postponed with Covid and latest date I can see on RERA site is DEC 2024. Due to some personal reasons I decided to cancelled the flat and informed the same to Marathon on email as well as on phone. They just sent me pre-cancellation letter in Nov 2019 and was not ready to proceed that further, nothing happened after that. this is almost more than 3 year, when I try to call them send a email or visit their office, they do not entertain me at all and not even ready to respond. They just say like you have to pay 20% cancellation charges and we will pay you back remaining amount when that flat will be sold to another customer, until that we cannot do anything. Is there a way I can get my amount back as its more than 3 years now without any payment or interest they are just using my money and not ready to discuss about further process.