C. F. Keil and Franz Delitzsch on Numbers 3:9

Num 3:9

Moses was also to give the Levites to Aaron and his sons. “They are wholly given to him out of the children of Israel:” the repetition of נתוּנם here and in Num 8:16 is emphatic, and expressive of complete surrender (Ewald, §313). The Levites, however, as nethunim, must be distinguished from the nethinim of non-Israelitish descent, who were given to the Levites at a later period as temple slaves, to perform the lowest duties connected with the sanctuary (see at Jos 9:27).