Biblical Commentary on the Old Testament, by Carl Friedrich Keil and Franz Delitzsh, [1857-78], at sacred-texts.com
The occasion for this law was a representation made to Moses and the princes of the congregation by the heads of the fathers' houses (האבות for בּית־האבות, as in Exo 6:25, etc.) of the family of Gilead the Manassite, to which Zelophehad (Num 26:33) belonged, to the effect that, by allotting an hereditary possession to the daughters of Zelophehad, the tribe-territory assigned to the Manassites would be diminished if they should marry into another tribe. They founded their appeal upon the command of Jehovah, that the land was to be distributed by lot among the Israelites for an inheritance (Num 36:2 compared with Num 26:55-56, and Num 33:54); and although it is not expressly stated, yet on the ground of the promise of the everlasting possession of Canaan (Gen 17:8), and the provision made by the law, that an inheritance was not to be alienated (Lev 25:10, Lev 25:13, Lev 25:23.), they understood it as signifying that the portion assigned to each tribe was to continue unchanged to all generations. (The singular pronoun, my Lord, in Num 36:2, refers to the speaker, as in Num 32:27.) Now, as the inheritance of their brother, i.e., their tribe-mate Zelophehad, had been given to his daughters (Num 27:1), if they should be chosen as wives by any of the children of the (other) tribes of Israel, i.e., should marry into another tribe, their inheritance would be taken away from the tribe-territory of Manasseh, and would be added to that of the tribe into which they were received. The suffix להם (Num 36:3) refers ad sensum to מטּה, the tribe regarded according to its members.
And when the year of jubilee came round (see Lev 25:10), their inheritance would be entirely withdrawn from the tribe of Manasseh. Strictly speaking, the hereditary property would pass at once, when the marriage took place, to the tribe into which an heiress married, and not merely at the year of jubilee. But up to the year of jubilee it was always possible that the hereditary property might revert to the tribe of Manasseh, either through the marriage being childless, or through the purchase of the inheritance. But in the year of jubilee all landed property that had been alienated was to return to its original proprietor or his heir (Lev 25:33.). In this way the transfer of an inheritance from one tribe to another, which took place in consequence of a marriage, would be established in perpetuity. And it was in this sense that the elders of the tribe of Manasseh meant that a portion of the inheritance which had fallen to them by lot would be taken away from their tribe at the year of jubilee.
Moses declared that what they had affirmed was right (כּן), and then, by command of Jehovah, he told the daughters of Zelophehad that they might marry whoever pleased them (the suffix ־הם, attached to בּעיני, for ־הן, as in Exo 1:21; Gen 31:9, etc.), but that he must belong to the family of their father's tribe, that is to say, must be a Manassite. For (Num 36:7) the inheritance was not to turn away the Israelites from one tribe to another (not to be transferred from one to another), but every Israelite was to keep to the inheritance of his father's tribe, and no one was to enter upon the possession of another tribe by marrying an heiress belonging to that tribe. This is afterwards extended, in Num 36:8 and Num 36:9, into a general law for every heiress in Israel.
In Num 36:10-12 it is related that, in accordance with these instructions, the five daughters of Zelophehad, whose names are repeated from Num 26:33 and Num 27:1 (see also Jos 17:3), married husbands from the families of the Manassites, namely, sons of their cousins (? uncles), and thus their inheritance remained in their father's tribe (על היה, to be and remain upon anything).
The conclusion refers not merely to the laws and rights contained in Num 33:50-36:13, but includes the rest of the laws given in the steppes of Moab (ch. 25-30), and forms the conclusion tot he whole book, which places the lawgiving in the steppes of Moab by the side of the lawgiving at Mount Sinai (Lev 26:46; Lev 27:34) and bring sit to a close, though without in any way implying that the explanation (בּאר, Deu 1:5), further development, and hortatory enforcement of the law and its testimonies, statutes, and judgments (Deu 1:5; Deu 4:44., Num 12:1.), which follow in Deuteronomy, are not of Mosaic origin.