Fraud in transferring the vehical

I bought a pickup from original owner mr X and after using sometime I sold it to another person mr Y and then the another person Mr Y sold it to Mr. Z , Now at the time when mr. Y bought the vehical from me he paid only half of the amount we were in urgent need of money so we had to made a deal. Now he used the van for 1.5 years and then he sold the van to another person Mr. Z without paying my Money . I didn't give the papers to neither Mr. Y nor Mr. Z . Now this person Mr. Z is a constable by proffession. He paid half the amount to Mr. Y and My portion of money which was remaining from Mr. Y was still not paid to me. Intitially we had some issues with the papers of van so Mr. Z said Give me the papers and I will give you the money . Due to covid there was a huge amount of problems and delays in paper processings. Nowthe papers are ready But Mr. Z the constable is not GIving me my money, He is saying he paid me in full in advanced now he is asking to handover papers. Please Help.