Forceful possession of car without completion of transfer formality

I have sold a car to a person six months ago. He has given me some advance (40%) . I gave him the car possession and all car documents. He agreed to give me balance amount after completion of transfer of ownership from my name to his name by him. However even after six months, he has neither completed the ownership transfer activity nor had he given me the balance amount. He is using the car having registration of my name as it is. I had repeatedly requested him to complete the activities however he is purposefully ignoring my request. Please let me know what legal action I can take against him.