Mon Sotah 21

If someone gives a present to Reuvain that should go to Shimon after Reuvain's death. R' Shimon b. Gamliel says if Reuvain sells it, Shimon cannot retrieve it after Revain's death. The Gemara calls someone a Rasha to give advice to Reuvain to sell it so Shimon won't receive it.

Tosfos asks: how do we reconcile that we hold like R' Shimon b. Gamliel, yet we hold like Reish Lakis that if someone only owns something for what it produce we don't consider him the true owner. Therefore, since Reuvain only has it for a limited time, we assume it's only his for the use he can have until his death. However, the object really belongs to Shimon, since he will eventually keep it. So since Reuvain is not the real owner, how can he sell the object for good?

Tosfos answers in the name of Rashbam: that it's not as we assumed, that Reuvain only owns what the object will produce until his death, but rather he receives the full item. Therefore it's his to sell. However, he has a limited ownership of it, until his death. Then the whole item reverts to Shimon.