Numbers 3:45

(45) And the cattle of the Levites instead of their cattle.—There appears to have been no numbering of the cattle. Had it been otherwise, an argument might have been used in support of the prospective reference of the command to number the firstborn, derived from the fact that it would have been impossible to ascertain the number of firstborn among the cattle. It appears, however, that the whole of the cattle of the Levites was given in redemption of the firstborn of all the cattle of the other tribes.

A Bible Commentary for English Readers, edited by Charles John Ellicott (1878–1897) — public domain. Text from a community e-Sword module.

And for those that are to be redeemed, &c. With money, there being not Levites enough to answer to them, and exchange for them:

of the two hundred and threescore and thirteen of the firstborn of the children of Israel, which are more than the Levites; for the sum total of the Levites, as given, was but 22,000, Num 3:39, whereas the sum total of the firstborn of Israel were 22,273, Num 3:43, so that there were, two hundred seventy three more of the latter than of the former; and what was to be done with these next follows.

John Gill's Exposition of the Whole Bible (1746–1763) — public domain (CC PDM 1.0). Text via the HelloAO Bible API.

Take the Levites, to wit, the 22,000 reckoned up Num 3:39, See Poole "Num 3:39"

Matthew Poole's English Annotations on the Holy Bible (1683–1685) — public domain. Text from a community e-Sword/MySword module.

ואת בהמת הלוים וגו׳ AND [THOU SHALT TAKE] THE BEASTS OF THE LEVITES [INSTEAD OF ALL THE FIRSTBORN AMONG THE BEASTS OF THE CHILDREN OF ISRAEL] — The beasts of the Levites did not redeem the firstborn of the clean beasts belonging to the Israelites, but only the first offsprings of their asses (cf. Bekhorot 4a). One lamb of the Levites (it must be remembered that for the redemption of פטר חמור a lamb is prescribed; cf. Exodus 13:13) could, however, release several first offsprings of asses belonging to Israelites. You may know that this is so, for it (Scripture) mentions the number of firstborn Israelites in excess of the number of Levites in the case of human beings, but mentions no number of firstborn animals belonging to the Israelites as being in excess over the lambs of the Levites, [although there were undoubtedly many more firstborn asses of the Israelites than ordinary lambs of the Levites] (cf. Bekhorot 4b).

Rashi on the Tanakh — English translations: “The Judaica Press complete Tanach with Rashi, translated by A. J. Rosenberg” (CC BY 4.0); “Pentateuch with Rashi's commentary by M. Rosenbaum and A.M. Silbermann, 1929-1934” (public domain); “The Metsudah Tanach series, Lakewood, N.J” (CC BY 4.0); and “The Book of Joshua, Metsudah Publications, 1997” (CC BY 4.0). Text via Sefaria.

TAKE THE LEVITES INSTEAD OF ALL THE FIRSTBORN AMONG THE CHILDREN OF ISRAEL. The firstborns had become sanctified to G-d from the time that He commanded Sanctify unto Me all the firstborn … among the children of Israel etc. There were thus many firstborns in Israel who had not been redeemed until now, since it had not yet been said who would “redeem” them, for it was only now that the priests became sanctified, and [Israel] had not yet been commanded about the gifts of the priesthood. Thus they [the firstborns] continued without restriction in their sanctity, and it is possible that they performed the Service of the offerings, as our Rabbis have said. The correct opinion appears to me to be that not all these firstborns were born in the desert in one year, for the people did not increase there so rapidly [that there should have been twenty-two thousand firstborns in one year, corresponding to the twenty-two thousand Levites, with an excess over the Levites of two hundred and seventy-three]. Rather, all firstborns of Israel [alive at that time, even those born many years previously] were counted, for they were all sanctified, as I have explained there. Now, He exchanged them for the Levites, and this was their “redemption,” and He commanded to redeem those that exceeded [the number of the Levites by means of each one giving five shekels]. And He gave the redemption-money to Aaron and his sons, as is the commandment for all generations [that a firstborn son be redeemed by giving five shekels to a priest].

Ramban (Nachmanides) on the Tanakh — English translation: “Commentary on the Torah by Ramban (Nachmanides). Translated and annotated by Charles B. Chavel. New York, Shilo Pub. House, 1971-1976” (CC BY 4.0). Text via Sefaria.