Albert Barnes Commentary Numbers 3:44-51

"And Jehovah spake unto Moses, saying, Take the Levites instead of all the first-born among the children of Israel, and the cattle of the Levites instead of their cattle; and the Levites shall be mine: I am Jehovah. And for the redemption of the two hundred and threescore and thirteen of the first-born of the children of Israel, that are over and above [the number of] the Levites, thou shalt take five shekels apiece by the poll; after the shekel of the sanctuary shalt thou take them (the shekel is twenty gerahs): and thou shalt give the money, wherewith the odd number of them is redeemed, unto Aaron and to his sons. And Moses took the redemption-money from them that were over and above them that were redeemed by the Levites; from the first-born of the children of Israel took he the money, a thousand three hundred and threescore and five [shekels], after the shekel of the sanctuary: and Moses gave the redemption-money unto Aaron and to his sons, according to the word of Jehovah, as Jehovah commanded Moses." — Numbers 3:44-51 (ASV)

This redemption money (see the marginal references) would perhaps be exacted from the parents of the “youngest” children of the 22,273 (Numbers 3:43). The cattle of the Levites was undoubtedly taken as a whole as an equivalent for the first-born cattle of the other tribes, which of course, no less than the first-born of men, belonged to the Lord; and in the future would have to be redeemed (Numbers 18:15); (Deuteronomy 15:19).