Someone claims the ownership of our property

We bought a flat in banglore in 2021 for 35 lakhs .we paid 3 lakhs and got private finance.Now some one claims that they paid 5 lakhs to the owner and got a registered agreement.The owner absconded ,so he wants us to pay 5 lakhs .This is the only property we have .So we agreed to pay him 5 lakhs to get away from local issues ,so now he is demanding 10 lakhs . That house was our only property .We dont have any income source .We already invested too much into this property .He claims that registrations occurred during covid time has 2 years extra validity ,so your ownership is illegal .Now we were even ready to sell the property to him .But he said he would buy that for 15 lakhs ,the amount they agreed during their agreement in 2018.What legal action can we take