Or HaChaim on Numbers 32:24/אור החיים על במדבר ל״ב:כ״ד

1 והיוצא מפיכם תעשו, "and do that which your mouths have uttered." Moses wanted them to understand that the permission he had granted for these tribes to build cities for their women and children etc., was not absolute but was conditional on their fulfilling the part of full participation with the other tribes in their conquest of the land of Canaan. It is also possible that Moses referred to the gratuitous offer by the two tribes to stay with the main body not only for seven years but for fourteen years until they had all been settled provided they were allowed to build fortified cities for their families. This is why he added the letter ו before the word והיוצא מפיכם, "and what your mouths have uttered." Having made an undertaking which Moses had not asked of them, they were to be held to it as it had been entirely voluntary. Moses made it plain that as far as his own conditions were concerned he had only obligated them to remain with the main body for seven years. Anything over and beyond that were conditions they had imposed upon themselves.