Or HaChaim on Numbers 32:29/אור החיים על במדבר ל״ב:כ״ט

1 ויאמר משה…ונתתם להם, Moses said:…"and then you will give to them, etc." The reason Moses had to say: "and you will give to them," was that it should be perceived that it was the other tribes who had given these lands to the tribes of Gad and Reuven although we will be reading in verse 33 that "Moses gave to the tribes of Gad, Reuven and half the tribe of Menashe the kingdom of Sichon and the kingdom of Og, etc." What Moses handed over became an absolute possession only after the tribes involved had completed their undertaking. We need to examine why Moses said: ונתתם להם instead of: תתנו להם, the more acceptable form of the future tense. Perhaps Moses wanted to indicate by the use of a form involving the letter ו at the beginning that the very whole-hearted participation in the conquest by these two tribes was what would make the campaign successful so that the ten tribes themselves owed their inheritance to that participation of the tribes Gad and Reuven. Had Moses written תתנו להם, we would not have surmised his meaning from that word.