Or HaChaim on Numbers 3:41/אור החיים על במדבר ג׳:מ״א

1 ואת בהמת הלוים, and the animals of the Levites, etc. The Torah speaks of those animals which the firstborn owned and which had been subject to redemption such as the firstborn donkeys. The Torah (Exodus 13,13) legislated that the firstborn pure animals had to be offered as a sacrifice whereas the firstborn donkey (being an unclean animal unfit for the altar) had to be redeemed. Firstborn animals of the pure species which for some reason did not qualify for the altar became the property of their owners and lost any status of sanctity they had had prior to becoming disqualified. We could have assumed therefore that the law of redeeming them did not apply when the swap between the Levites and the firstborn Israelites was made. Hence the Torah had to go on record that firstborn donkeys owned by the firstborn being exchanged for a Levite had to be redeemed.