When DM and subsequently DF passed, DW got her engagement ring. The ring band itself was well worn and not enough was left to adjust to fit DW's fingers. In order for the family heirloom to be carried on, we had the mount with diamond (with 2 smaller diamonds, chips actually) cut free and attached to a much wider band. When DS was planning to get engaged, he asked if he could have the ring as her engagement ring. Of course we said YES! Afterwards, they decided the ring was not her, but they really wished to carry on the family history. Would we allow them to remove the diamond and have it remounted? Of course. It was theirs. We did ask that if they weren't going to use the band/mount, would they please return it? Which they did. I had another stone mounted where the diamond was originally and DW is very happy.
I know this is not an answer to the original question. I agree with what most everyone has posted. It would be a gift to your son who then gifts it to his betrothed. There is no backsies 1 step removed. One can hope that this will never become an issue and DS and DDIL will live happily ever after. In our case, if DS's marriage ends and if the engagement ring (diamond) is not returned, at least we still have some of the original in the family.