He prohibited us from redeeming a pure firstborn animal. And that is His, may He be exalted, saying, "But the firstborn of cattle, etc. may not be redeemed" (Numbers 18:17). But it is permitted to sell it, as it is explained in Bekhorot (Bekhorot 37b). And the language of the Sifra (Sifra, Bechukotai, Section 5:3), is, "With the firstborn, it is stated, 'it may not be redeemed' - but it may be sold." (See Parashat Korach; Mishneh Torah, Firstlings 1.)