Alternate Answer (Rava): It is speaking where the mother of the Sa'ir was slaughtered on that day (for an ill person) thus creating the prohibition of Oso ve'Es B'no.

Question: But we are not doing Shechitah on the child on the same day as the mother, we are only pushing it off a cliff!?

Answer: For the Sa'ir ha'Mishtale'ach, that is its Shechitah.

(Rav): The remaining Sa'ir from the original pair is used, and the match to the new Sa'ir is put out to pasture.

(R. Yochanan): The opposite (original is pastured, new one is used).

Question: What is the basis for their dispute?

Answer: Rav holds that the principle of Dichui does not apply prior to Shechitah; while R. Yochanan holds that Dichui does apply to Ba'alei Cha'im.

Question: Why does Rav hold his position?

Answer: He learns it from the usual case of Mechusar Z'man, which will become fit even though at the moment it is not fit.

Question: But by Mechusar Z'man the animal never went through a period in which it was fit, unlike our case?!

Answer: Rather, Rav compares it to the case of an animal with a temporary blemish.

Question: Whence do we know that such an animal may be subsequently offered?

Answer: From the Pasuk "Mum Bam" (only while the blemish is upon them may they not be offered).

Question: Why does R. Yochanan hold his position (and not learn, as Rav did, from an impermanent Mum)?

Answer: Because the Torah restricted its Heter there with the word "Bahem" (Mum is the only Nidcheh which comes back).

Question: How will Rav understand "Bahem"?

Answer: "Bahem" teaches that the Isur of Mum only applies if the limb of the Ba'al Mum can be identified, but not in a mixture (as we see is R. Eliezer's position in the Mishnah in Zevachim).

Question: Whence then will R. Yochanan learn this Din?

Answer: The word "Bahem" could have otherwise read 'Bam'.

Question: How will Rav understand 'Bam'-"Bahem"?

Answer: Rav does not interpret the difference between them.

Question: Why, according to Rav, must we offer the original Sa'ir, why not offer either one?

Answer: Rav holds like R. Yosi that the proper Mitzvah is with the first one designated.

Question: Where is this opinion of R. Yosi?

Answer: When R. Yosi explains the requirement to mark the Kupos as in order to use the first one first.

Question: But that may be required since the second has not yet become usable when the first Kupah was available, but here all the Se'irim become available at the same moment (conclusion of the Matnos Damim)!?

Answer: Rather, the source is R. Yosi regarding the Korban Pesach which was lost, replaced and then found where R. Yosi holds that the first is to be used, unless the second one is superior to it.

(Rava): The Mishnah seems to imply Rav's position while the Beraisa seems to imply R. Yochanan's.

The Mishnah says that the new animal la'Shem stands in the stead of the one which died, but does not say that the new Mishtale'ach stands in stead of the first (like Rav).

The Beraisa teaches explicitly that the Sheini is the new Sa'ir, whose partner had not died.

R. Yehudah (also) taught in our Mishnah that the Sa'ir ha'Mishtale'ach must be put to death if the blood la'Shem spilt.

Question: While this is understandable according to R. Yochanan (Ba'alei Cha'im Nidachin), why, according to Rav is this Ba'al Chai Nidcheh?

Answer: Rav admits that he is not teaching his position in R. Yehudah, but rather in the Rabanan (see Rashi for how the Rabanan's position implies that Ba'alei Cha'im are not Nidachin).

Question: Rav's position explains R. Yehudah's argument with the Rabanan, but according to R. Yochanan, what is their argument (they both speak of the same animal)?

Answer (Rava): That is (another reason) why we said earlier that the Mishnah implies the position of Rav.