Hello and welcome to JustAnswer. Please note: This is general information for educational purposes only and is not legal advice. No specific course of action is proposed herein, and no attorney-client relationship or privilege is formed by speaking to an expert on this site. By continuing, you confirm that you understand and agree to these terms.
I am sorry to hear about this situation. The contract was finalized at the time of the transaction. The fact that the seller lost (or so he claims) the check does not matter. So the transaction is FINAL.
Now, you can place a stop payment on the cashier's check and reissue another one. And this is recommended, as it is the ethical thing to do and honestly, the seller can sue to enforce this. But the seller may still be responsible for the stop payment amount.
But aside from what was agreed payment on the car (again, the original cashier's check may be stopped, and then reissued and a fresh copy given to him), nothing else needs to be paid.
I hope this helps and clarifies. Please use the SEND or REPLY button to keep chatting, or please RATE when finished. You may always ask follow ups at no charge after rating. Kindly rate my answer as one of TOP THREE FACES/STARS and then SUBMIT, as this is how experts get credit for our time. Rating my answer the bottom two faces/stars (or failing to submit the rating) does not give me credit and reflects poorly on me, even if my answer is correct. I work very hard to formulate an informative and honest answer for you; please reciprocate my good faith with a positive rating.