Why did they mention the Yovel?
Rashi: They pointed out that unlike a sale, where the land reverts to the original owner in Yovel, and where the problem would not exist, inheritance does not go back in the Yovel.
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 the 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.
Why does the Torah insert the word "ve'Im Yihyeh ha'Yovel"?
Rashi (citing Toras Kohanim): R. Yehudah learned from here that the Yovel cycle will be broken. 1
Rashi (in Rosh Hashanah, 3a): "ve'Im" here means (not 'if' but) 'when', since the Yovel is not a Safek; it is bound to occur. 2
Why does the Torah insert the word "ve'Im Yih'yeh ha'Yovel"?