You had a contract for the purchase of a ring.  That contract was verbal.  That is ok is most instances and here it is probably fine. If the contract was based upon fraud or misrepresentation or mistake, it is void.  Here is was a mistake so he can ask for his money back, which he has.  Harassing and threatening you, though, are not the legal ways to get return of his money.  Suing you is.  But I do not think that he wants to go that route.  So return his money and get the ring back.  Then have it properly appraised so that next time you do not run in to this problem.  Good luck.