Rashi on Leviticus 27:11/רש"י על ויקרא כ״ז:י״א

1ואם כל בהמה טמאה AND IF IT BE ANY בהמה טמאה — Scripture is really speaking here not of an “unclean" but of a blemished animal which ,is just on this account, unclean (i. e. unfit) for sacrificing, and Scripture is telling you that sacred animals which have no blemish cannot again become non-holy (more lit., go forth from the category of holy animals to that of non-holy animals) through redemption except if they become blemished (Temurah 32b; Sifra, Bechukotai, Section 4 1).