There is NO way you should give him back that ring.
Well, there IS one way I could imagine, and it's probably an indicator of how evil I am.
1) Hire a concrete mixer truck.
2) Take video of you throwing engagement ring into concrete mixer.
3) Pour concrete in a location that won't get you arrested. Wait a couple days so it can set.
4) Tell him the ring is somewhere in there (Point at pile of concrete).
I know this is a complete and utter waste of money, and completely impractical, so selling it is the better course. Sigh.
That said... maybe sell it here... or at least browse through the stories.
Jewelry Archives - Never Liked It Anyway