John Calvin Commentary Leviticus 27:26

"Only the firstling among beasts, which is made a firstling to Jehovah, no man shall sanctify it; whether it be ox or sheep, it is Jehovah`s." — Leviticus 27:26 (ASV)

Only the firstling of the beasts. Here a caution is introduced, that no one should offer what is already the property of God. For since people are so prone to ostentation, and therefore in demonstrating their piety whitewash two walls, as the saying goes, out of the same pot, God guards against this sin by forbidding the first-born to be offered to Him, since that would be to present stolen goods to Him.

In essence, they should not, by consecrating to God what is already due to Him, steal from Him through their false generosity what is consecrated and not their own. And we should not be surprised by this law, because this ambition is almost natural to all of us: to desire to put God under an obligation through the empty appearance of generosity, and therefore to seek various reasons for boasting about religious duties that, after all, are nothing. And, undoubtedly, if this restraint had not been imposed on the Jews, they would have sought a reputation for double zeal through this deceitful offering, nor would they have hesitated, under the pretext of offering, to deprive God of what was His own.