Temurah 23B

מאי לאו אפילו רבנן? לא, רבי היא.

The Talmud concludes: What, is it not that this Mishnah is stated even in accordance with the opinion of the Rabbis? If so, the Mishnah is teaching that even according to the Rabbis, an offering designated in the stead of a lost animal is considered like the lost animal, which would contradict the statement of R' Abba. The Talmud responds: No, it is stated only in accordance with the opinion of R' Yehuda HaNasi.

תנן: המפריש חטאת ואבדה, והקריב אחרת תחתיה ⁠— תמות.

The Talmud poses a challenge to the explanations of both Rav Huna and R' Abba from that which we learned in the Mishnah: In the case of one who designates a sin offering, and the animal was lost, and he designated another in its stead and sacrificed it, and the first animal was subsequently found, that is a sin offering whose owner achieved atonement with another animal, and it should be left to die.

טעמא (דקריבה) [משום דהקריבה], הא לא הקריבה ⁠— תרעה, לא שנא מתכפר באבודה, לא שנא מתכפר בשאינה אבודה, לא שנא משך, ולא שנא לא משך.

The Talmud infers: The reason that the original animal is left to die is that the second animal was already sacrificed, but if the owner did not sacrifice the second animal before the original animal was found, then the original animal is left to graze. The Mishnah apparently indicates that there is no difference whether he achieves atonement with the sin offering that was lost and there is no difference whether he achieves atonement with the sin offering that was not lost. It can likewise be inferred from the Mishnah that there is no difference whether he took the animal and sacrificed it without consulting the court and there is no difference whether he did not take the animal and sacrifice it without consulting the court. In all cases, if the owner had not yet achieved atonement then the remaining animal is left to graze.