I would stress the sentimentality that the proposal ring has, many women are very attached to it. And also point out that you can replace it sometime in the future since he was planning on an upgrade at some point anyway (maybe a couple of years sooner than planned?) Since he didn’t buy from a vendor with an upgrade policy he obviously wasn’t planning on doing the upgrade by getting the full value out of the original diamond anyway. Most couples who upgrade but don’t have an upgrade policy end up buying a second ring and keeping their first for sentimental value since their first cant be sold or traded in for any significant amount.