Charles Ellicott Commentary Numbers 36:4

"And when the jubilee of the children of Israel shall be, then will their inheritance be added unto the inheritance of the tribe whereunto they shall belong: so will their inheritance be taken away from the inheritance of the tribe of our fathers." — Numbers 36:4 (ASV)

And when the jubile of the children of Israel shall be. —Up to the year of jubile it was possible that the inheritance might revert to the tribe of Manasseh, either by purchase, or as the result of the marriages of the daughters proving childless. At the jubile the transfer of the inheritance to the tribe or tribes into which the daughters of Zelophehad might have married would become permanent.