Rashi on Leviticus 27:9/רש"י על ויקרא כ״ז:ט׳

1כל אשר יתן ממנו EVERYTHING THAT A MAN GIVE implies even the case when he said, “the foot of this animal shall be a burnt offering” his words have binding force). It (the animal) is therefore sold for the purpose of a burnt-offering to one who has to bring such an offering and the monies realized by its sale are חולין (non-holy) and are returned to the owner with the exception of the value of that limb (Sifra, Bechukotai, Chapter 9 1; Arakhin 5a; Temurah 11b; Chullin 69b).