Bekhorot 33B

ולוקמה כולה כרבי אלעזר ברבי שמעון! דילמא עד כאן לא קאמר רבי אלעזר ברבי שמעון התם אלא פסולי המוקדשין, דאלימי למיתפס פדיונו, אבל בכור דלא אלים למיתפס פדיונו ⁠— לא.

The Talmud suggests: But if so, let Rav Ḥisda establish the entire baraita in accordance with the opinion of R' Elazar, son of R' Shimon. The Talmud answers: Perhaps R' Elazar, son of R' Shimon, says that the sanctity of a slaughtered blemished offering is retained only there, with regard to other disqualified consecrated animals, because their level of sanctity is strong enough to transfer their sanctity to their redemption money. But with regard to a blemished firstborn offering, whose level of sanctity is not strong enough to transfer it to its redemption money, perhaps its sanctity is not retained, and therefore one may skin the hide from its feet.

ורבי אלעזר ברבי שמעון לית ליה כל פסולי המוקדשין נמכרין באיטליז ונשקלין בליטרא? אלמא דכיון דאיכא רווחא להקדש שרי לה!

The Talmud asks: But doesn’t R' Elazar, son of R' Shimon, agree with the ruling of the Mishnah (31a) that all disqualified consecrated animals are sold in the butchers’ market and weighed and sold by the litra, in the manner of non-sacred meat? Certainly he agrees with that halakha. Evidently, since there is a benefit that accrues to the Temple treasury, the tanna of the Mishnah deems this permitted. As it was taught (31b), the meat of disqualified consecrated animals may be treated in this manner despite the fact that it retains a measure of sanctity. The reason is that if the owner knows that it is permitted for him to perform this more lucrative action, he is likely to spend more money to redeem the animal in the first place, which benefits the Temple treasury. By the same logic, why doesn’t R' Elazar, son of R' Shimon, permit the more lucrative action of skinning the animal from its feet?