Post breakup, should I return the ring?

My boyfriend recently left me and, sparing all details, it was a bit of a shocker. Over the course of the relationship, he'd given me a promise ring because I wasn't myself ready for an engagement (I know, promise rings are a bit juvenile, but I'm the one that had the commitment-phobia). At one point, when things were fine between us, it randomly came up in conversation what would happen to the ring. He said that it was a gift to me from him, and it would upset him to get it back. Now, I don't have any reason for the ring, it was a promise from him, and I'm painfully reminded of his broken promise every time I see it. We've had no contact, and I'm mailing the rest of his stuff to him. This hasn't been a messy breakup, no nasty words, shouting matches, etc. It just ended. My goal isn't to hurt him by giving the ring back, I just don't know what to do with it myself.