Сифрей Мицвот к Вайикра 27:10

He prohibited us from substituting consecrated animals. And that is His saying, "One may not exchange nor substitute for it" (Leviticus 27:10). And the prohibition already appeared specifically about the [animal] tithe. And the reason is that which they mentioned in the [Sifra] (Sifra, Bechukotai, Chapter 9:4): And that is their saying, "The tithe was in the category [of all consecrated animals]. Why, then, was it singled out? To make a comparison to it: Just as the tithe is consecrated animals that go on the altar and its substitution is not permitted; so too, is the substitution of all consecrated animals that go on the altar not permitted." And that is His saying, "nor substitute for it," and His saying, "One may not exchange" - however that is only with consecrated animals that go on the altar. And one who substitutes [it] is lashed. And the regulations of this commandment have already been explained in Temurah. (See Parashat Bechukotai; Mishneh Torah, Substitution 1.)