Sforno on Numbers 8:16/ספורנו על במדבר ח׳:ט״ז

1כי נתונים נתונים המה לי, they have devoted themselves to me and My service, as the Torah testified in Exodus 32,26 when all of them responded to Moses’ call that whoever was for the Lord should assemble around him. מתוך בני ישראל, they are also “נתונים” by the Children of Israel who will provide them with a livelihood by means of giving them their tithes, i.e. the מעשר ראשון 10% of their harvest after hey had set aside the t’rumah for the priests.

2תחת פטרה כל רחם, who had originally been assigned the duties now assumed by the Levites.