Комментарий к Вайикра 27:26

לא יקדיש איש אתו [ONLY THE FIRSTBORN OF BEASTS…] NO MAN SHALL SANCTIFY IT for the purpose of (to become) a sacrifice of any other description, for it is not his, [being already holy to the Lord from birth] (cf. Arakhin 29a).

ONLY THE FIRSTBORN OF THE BEASTS … NO MAN SHALL SANCTIFY IT — “as a different offering, because it is not his.” Thus the language of Rashi. The correct interpretation is that He is saying: “there is no need to sanctify it [the firstling], for whether it be ox or sheep, it is the Eternal’s automatically.” The verse is thus [to be understood] as a negation.¹⁹³See in Exodus. Vol. II, p. 350, Note 71. And in the Book of Deuteronomy He stated, All the firstling males that are born of thy herd and of thy flock thou shalt sanctify unto the Eternal thy G-d,¹⁹⁴Deuteronomy 15:19. This verse does not mean that the owner is to declare it “holy,” since it is automatically holy by virtue of being the firstborn, but, as Ramban continues, that we are to regard it as holy, etc. meaning that it is holy and that you are to treat it with holiness, so that you are not to do work with it, nor shear it,¹⁹⁴Deuteronomy 15:19. This verse does not mean that the owner is to declare it “holy,” since it is automatically holy by virtue of being the firstborn, but, as Ramban continues, that we are to regard it as holy, etc. and that you are to eat it before the Eternal,¹⁹⁵Ibid., Verse 20. as He will explain.

לא יקדיש איש אותו, no one can declare it as sanctified to be a different category of sacrifice. Seeing it had been born sanctified, the owner cannot change its status.