Car ownership regarding

Hai Kaanoonians Good evening, I'm Ashok from Coimbatore, I bought a second hand car from a car dealership which is park and sell type. Dealer will allow you to park the car in his facility and he will find buyer then he will connect buyer and seller through his phone only. Once price finalized dealer will get Rs.5000 from buyer and Rs.5000 from seller as commission and he will get papers from seller and will provide it to buyer No face to face communication between buyer and seller (If buyer and seller meets face to face dealer will not get any commission as both parties will try to avoid paying dealer fee) I bought car on March 2018 and transferred car ownership To my name Today suddenly a lady approached me when I was waiting at single and said this car belongs to her, she gave car to her relative to some repairs but he sold the car without her knowledge. She asked me to coordinate with her for legal proceedings in a polite manner. I too agreed and I gave my mobile number and the car dealer ship mobile to her. In the shock I forget to get her number Can u please suggest what can I do now. I don't have the payment receipt which I paid to the dealer I thought it was not important and lost it