You need to accept that nothing you can do will hurt him. He is not build like you, his emotions aren't the same as yours, he doesn't think and react like you.
If you are thinking keeping the ring solely to case him emotional pain, then you are fighting a battle for nothing. He revels in this kind of battle. It's his oxygen, his water! And you will be supplying him.
If you want to keep it for a financial gain and are prepared for the battle that lies ahead, then go for it. But it's still a battle he wants. He doesn't care if he wins the ring in the end, it's the battle that counts to him.
There will be more solicitor's letters, he may even drag it to court. Again he doesn't care about winning. He just enjoys the progress because to him he is still controlling you and keeping you hooked into the drama.
Keep it or sell it, but be sure you're doing it for the right reasons and have the stamina for a fight.
The opposite of love is not hate, it's apathy. And you need to find your apathy. Hating him doesn't hurt him. It hurts you.