Adam Clarke on Numbers 18:28

Thus ye also shall offer a heave-offering - As the Levites had the tithe of the whole land, they themselves were obliged to give the tithe of this tithe to the priests, so that this considerably lessened their revenue. And this tithe or tenth they were obliged to select from the best part of the substance they had received, Num 18:29, etc. A portion of all must be given to God, as an evidence of his goodness, and their dependence on him. See the end at Num 20:28.