Rashi on Numbers 36:4/רש"י על במדבר ל״ו:ד׳

1ואם יהיה היבל may mean AND IF THERE WILL BE THE JUBILEE — From this R. Judah said: The observance of the Jubilee will cease at some time or other (Sifra, Vayikra Dibbura d'Nedavah, Section 13 1 on Leviticus 2:14).

2ואם יהיה היבל may mean, too: AND WHEN THERE WILL BE THE JUBILEE [THEN WILL THEIR INHERITANCE BE ADDED TO THE INHERITANCE OF THE TRIBE TO WHICH THEY SHALL BELONG] — meaning: This is no sale of land so that it returns to the original owner at the Jubilee, for a heritage never returns. Therefore even when the Jubilee will be, the inheritance will not return to its tribe, and it will follow that it will be added על נחלת המטה אשר תהיינה להם TO THE INHERITANCE OF THE TRIBE TO WHICH THEY SHALL BELONG.