Leviticus 27:26

“Only the firstlings of the beasts, etc.] These are excepted from being sanctified, or set apart for sacred uses, for a very good reason, suggested in the next clause: which should be the Lord’s firstling, no man shall sanctify it; it being what he has a claim upon, and ordered to be sanctified to him by a law previous to this, (<021302>Exodus 13:2); wherefore to sanctify such a creature, would be to sanctify what was his before; not merely in a general sense, in which all creatures are his, but in a special sense, having in a peculiar manner required it as his; and therefore to sanctify, or vow to him, what was his before, must be trifling with him, and mocking of him: whether [it be] ox, or…”

“Only the firstling of the beasts--These, in the case of clean beasts, being consecrated to God by a universal and standing law (Exo 13:12; Exo 34:19), could not be devoted; and in that of unclean beasts, were subject to the rule mentioned (Lev 27:11-12).”

“No man shall sanctify it - By vow; because it is not his own, but the Lord's already, and therefore to vow such a thing to God is a tacit derogation from, and an usurpation of the Lord's right, and a mocking of God by pretending to give what we cannot withhold from him. Or ox or sheep - Under these two eminent kinds he comprehends all other beasts which might be sacrificed to God, the firstlings whereof could not be redeemed but were to be sacrificed; whereas the firstlings of men were to be redeemed, and therefore were capable of being vowed, as we see, Sa1 1:11. Leviticus 27:27”

“27:26 You may not dedicate: Every firstborn male, be it man or animal, already belonged to God (Exod 13:2; 34:19-20). Therefore, dedicating a firstborn was a useless exercise. The priest clearly had a use for the firstborn of animals fit for food or sacrifice (clean animals). The firstborn of a ceremonially unclean animal such as a donkey could be redeemed for 20 percent more than its value. Human firstborns had to be redeemed (Exod 13:11-13).”

“the firstling: Heb. first born, etc. As these firstlings were the Lord's before, it would have been a solemn mockery to pretend to make them a matter of a singular vow; for they were already appointed, if clean, to be sacrificed. which: Exo 13:2, Exo 13:12, Exo 13:13, Exo 22:30; Num 18:17; Deu 15:19 Leviticus 27:27”

“What belonged to the Lord by law could not be dedicated to Him by a vow, especially the first-born of clean cattle (cf. Exo 13:1-2). The first-born of unclean animals were to be redeemed according to the valuation of the priest, with the addition of a fifth; and if this was not done, it was to be sold at the estimated value. By this regulation the earlier law, which commanded that an ass should either be redeemed with a sheep or else be put to death ( Exo 13:13; Exo 34:20), was modified in favour of the revenues of the sanctuary and its servants. Leviticus 27:28”