Comentario sobre Números 32:30

ואם לא יעברו חלוצים אתכם, "But if they will not cross with you, armed, etc." The emphasis in this verse is on the words אתכם, "with you," i.e. at the same time as you. If these two tribes were to decide to join you later and help in the conquest they would already have forfeited their claim to special consideration on the East Bank. The reason the Torah wrote ונאחזו בתוככם, "and they will have possessions amongst you," with the added letter ו at the beginning of the word ונאחזו instead of writing יאחזו, "they will receive a possession," is to point out that in such an event they have forfeited their success in this part of the land and they will automatically only inherit their share on the West Bank. They will have failed to perform the מצוה they took upon themselves and therefore will not have attained what they aimed for.

Then they shall take possession among you in the land of Canaan. Only in the land of Canaan, but they will receive no portion at all on the eastern side of the Yardein. This is the plain meaning of the text. However, the Talmud explains that “among you” is on the eastern side of the Yardein as well as in the land of Canaan. The Sages derived this because Scripture does not simply say, “Then they shall take possession in the land of Canaan.” The phrase “among you” comes to include the land across the Yardein (Kiddushin 61).