Bekhorot 26A

הא בעל מום שרי, אף על פי שלא התירו מומחה! תנא: כל היכא דלא התירו מומחה ⁠— תם קרי ליה.

But in the case of a blemished firstborn, use of its wool is permitted, even though an expert did not deem the animal permitted for slaughter. This contradicts the statement of R' Asi. The Talmud answers: It is possible that the tanna of that baraita calls the animal unblemished wherever an expert did not yet deem the firstborn permitted for slaughter, even if it was physically blemished.

לימא כתנאי: התולש צמר מבכור תם, אף על פי שנולד בו מום ושחטו ⁠— אסור. בכור בעל מום שתלש ממנו, ואחר כך מת ⁠— עקביא בן מהללאל מתיר, וחכמים אוסרים. אמר רבי יהודה: לא בזה התיר עקביא בן מהללאל, אלא בשער בכור בעל מום שנשר והניחו בחלון, ואחר כך שחטו ⁠— בזה עקביא בן מהללאל מתיר, וחכמים אוסרין.

The Talmud remarks: Let us say that the interpretation of Reish Lakish is the subject of a dispute between tanna’im, as the baraita states: With regard to one who plucks the wool from an unblemished firstborn, even though it later developed a blemish and the owner slaughtered it, use of the wool is prohibited. In the case of a blemished firstborn from which one plucked wool, and it subsequently died, Akavya ben Mahalalel deems the wool permitted and the Rabbis deem it prohibited. R' Yehuda said: It was not with regard to that case that Akavya ben Mahalalel deemed use of the wool permitted. Rather, it was in a case of the hair of a blemished firstborn animal that shed from the animal and one placed the hair in a compartment, and after that one slaughtered the animal. In that case Akavya ben Mahalalel deems use of the wool permitted, and the Rabbis deem its use prohibited.

אמר רבי יוסי: מודה אבא חלפתא בזה שהוא מותר, אבל בייחוד אמרו חכמים: יניחנו בחלון, שמא יש תקוה. שחטו ⁠— דברי הכל מותר, מת ⁠— עקביא בן מהללאל מתיר וחכמים אוסרים.

The baraita continues: R' Yosei says: My father, Ḥalafta, concedes to Akavya ben Mahalalel in this case of a slaughtered firstborn that use of the wool is permitted. Indeed [aval], the Rabbis said explicitly: One places it in a compartment, as perhaps there is a hope that use of the wool will ultimately be permitted, since if he slaughters the animal all agree that use of the wool is permitted. But if the animal dies by other means, Akavya ben Mahalalel deems use of the wool permitted and the Rabbis deem it prohibited.