Take it of them, that they may be to do the service of the tabernacle of the congregation; and thou shalt give them unto the Levites, to every man according to his service. (Numbers 7:5)
The wagons and oxen were a voluntary offering (v. 3) that must have been received gratefully by the Levites for the work of transportation. Moses apparently did not accept the offer until he was specifically authorized by the Lord to do so.