John Calvin Commentary Numbers 32:28

"So Moses gave charge concerning them to Eleazar the priest, and to Joshua the son of Nun, and to the heads of the fathers` [houses] of the tribes of the children of Israel." — Numbers 32:28 (ASV)

So concerning them Moses commanded. Moses attaches these conditions to his decision. He did this so that, when the Reubenites and their companions had performed their military tasks, they would not be falsely claimed to have passed over Jordan for the purpose of seeking a new home. Furthermore, if they were to deceive the other tribes, he ensured that their cowardice and deceit would not benefit them.

In short, if they assisted their brothers according to their agreement, he commands that the territory he now grants them should always remain theirs. However, if they broke their promise, he would have them forced against their will to participate in the common allotment.

For he does not assign them this portion in the midst of Canaan as a reward for their inaction, should they stay behind. Instead, he indicates that they would be forcibly and authoritatively made to go forward, so that they would be subject to their brothers under all circumstances, since it was not lawful for them to pursue their own separate interests.

In laying down rules for the division of the land, as if it were soon to happen, he encourages all to be confident, so that they would more cheerfully hasten to pass over; as if victory were not only already in their hands, but also that its fruits were soon to be enjoyed.