I did read about it. The very situation looks somewhat insane. If at the official meeting you pass something valuable to another party it normally means an official gift. The practice of official gifts is a part of international diplomacy tradition. AFAIK there is a giant and thoroughly documented depository of gift items in Kremlin (of accepted gifts as well as of valuables specifically crafted or selected for future gifting). Any official that makes any kind of international negotiation is obliged to transfer accepted gifts there, because it is a property of the state rather than a personal gift.

If that billionaire guy enjoy dealing with high rank diplomatic officials he should know very well about those mechanisms. And if he in several yeard starts making broadcast claimings about stealing... Well, I believe he risks to be silently banned from diplomatic events worldwide.

On the other hand how did it happen that Putin "doesn't remember" about that ring? It should be mentioned in the protocol, listed in the inventory and kept in the depository. Or... did he in fact steal that ring from Russian Federation?