John Calvin on Leviticus 27:26-29

26. Only the firstling of the beasts. Here a caution is interposed, that none should offer what is already the property of God. For since men are so greatly given to ostentation, and therefore in testifying their piety whitewash two walls, as the saying is, out of the same pot, God provides against this sin by forbidding the first-born to be offered to Him, since that would be to present stolen goods to Him. The sum is, that they should not, by consecrating to God what is already due to Him, steal from Him in their fictitious liberality what is consecrated and not their own. Nor let us be surprised at this law, because this ambition is almost natural to us all, to desire to lay God under obligation by the empty appearance of liberality, and therefore to seek for various grounds of boasting of religious duties, which, after all, are nought. And, undoubtedly, if this restraint had not been put upon the Jews, they would have aimed at the reputation of double zeal by this deceitful oblation, nor would they have scrupled, under the pretext of offering, to deprive God of what was His own.

Deuteronomy 15

Deuteronomy 15:19, 20

19. All the firstling males that come of thy herd, and of thy flock, thou shalt sanctify unto the Lord thy God: thou shalt do no work with the firstling of thy bullock, nor shear the firstling of thy sheep:

19. Omne primogenitum quod nascetur in armento, aut in grege tuo, masculum sanctificabis Jehovae Deo tuo: non uteris opera primogeniti bovis tui, neque tondebis primogenita gregis tui.

20. Thou shalt eat it before the Lord thy God year by year, in the place which the Lord shall choose, thou and thy household.

20. Coram Jehova Deo tuo comedes eum annuatim in loco quem elegerit Jehova, tu et familia tua.