Albert Barnes Commentary Numbers 18:29

"Out of all your gifts ye shall offer every heave-offering of Jehovah, of all the best thereof, even the hallowed part thereof out of it." — Numbers 18:29 (ASV)

Out of all your gifts - The spirit of this law would extend to all the revenues of the Levites; from the increase of their cattle, as well as from their tithes, a tithe would be paid by them for the Lord’s service.