Sforno on Numbers 3:9/ספורנו על במדבר ג׳:ט׳

1נתונים נתונים המה לו, all their (the Levites’) service was to be performed at the instruction of Aaron, to whom the Levites had been delegated by the people.

2מאת בני ישראל, who in turn obligated themselves to compensate their delegates, the Levites, by giving them the tithes of their crops.