Why did they mention the Yovel? Why might that have alleviated their problem?
Rashi: They mentioned Yovel, because in the case of a sale, the land reverts to the original owner, in which case the problem would not exist.
Seforno: The Torah is referring to a later stage, when members of a certain tribe have sold part of their inheritance to members of another tribe before it has been captured, they will not hesitate from going out to fight for it, knowing that, when Yovel arrives, it will be returned to them, which is not the case in current situation, where, in the event that the B'nos Tz'lofchad marry into another tribe, the members of Menasheh will refrain from fighting for that territory, which will not be returned to them - thereby causing a loss to the national inheritance.
Then why didn't it?
Rashi: Because it is only with regard to a sale that the land reverts to its original owner, but not when it comes to inheritance, where the heirs inherit permanently.
Why does the Torah insert the word "ve'Im Yih'yeh ha'Yovel"?