Komentaryo sa Levitico 27:10: Rashi, Rabbeinu Bahya, Chizkuni & Siftei Chakhamim

טוב ברע [HE SHALL NEITHER CHANGE IT NOR EXCHANGE IT] A GOOD FOR A BAD — i. e. he shall not give in exchange a perfect animal of a non-sacred character for one devoted to the Sanctuary but having some blemish (cf. Rashi on Temurah 9a on ‎‎‎מתני' טוב ברע),

לא יחליפנו ולא ימיר אותו טוב ברע או רע בטוב, “he shall not exchange it or substitute it, good for bad or bad for good.” According to Maimonides Hilchot Temurah 4,13 the reason for this legislation is that the Torah explored the psyche of the donor down to its ultimate depth. At the core of a human being, even the finest, there may still be lying in wait for him the evil urge. Even though the person in question made these vows entirely voluntarily, not in response to any pressure, he may yet at some time or other rue having made such a vow. His first attempt at getting out of his obligation would be to exchange the animal designated for the altar with an inferior one. Seeing that the animal he sanctified was suitable as an offering, the sanctity becomes part of its body and it cannot be exchanged. This is why the Torah stated flat out that such an animal as described in our verse לא יחליפנו, cannot be traded at all. If the Torah were to permit the donor to upgrade the animal in question, it would not be long before people would abuse such a rule and they would start downgrading it for an inferior beast. If the donor violated this legislation by substituting, not only would he not gain thereby but he is punished by the Torah which states that the substitute will also become sanctified thus depriving the donor of both animals.
והיה הוא ותמורתו קדש, “and both it (the original) and its substitute will remain sacred.” Keeping in mind the reason Maimonides advanced for this legislation, we can now understand why the Torah demanded that if someone who had sanctified his house for the Temple Treasury [a lesser degree of sanctity as the house could not be offered as an offering on the altar, Ed.] and he wants to redeem it from the Temple treasurer offering to pay for it, must add 25% as a premium to its value. The Torah’s reading of our minds allows for the fact that we may change our minds. In order to remind us that such a change of mind is not appreciated, the Torah imposes a price on that privilege of a person to change his mind (compare Nachmanides).

טוב ברע או רע בטוב, “(substituting) good for inferior, or inferior for superior. The reason why the Torah had to write both alternatives is because the owner does so as he is not sure which is superior and which is inferior. In the case of the animals proving to be diseased and therefore at least one of them being disqualified as an offering, the donor did not know which of the sicknesses is the more serious one. (B’chor shor)

‎‎‎או רע בטוב‎ OR A BAD FOR A GOOD — It follows à fortiori that he must not give in exchange a perfect animal for another perfect one, or one with a blemish for another with a blemish (Temurah 9a).