פירוש על ביצה 3:4

בכור שנפל לבור – a firstling with a blemish, which had not been shown to a Sage from the eve of the Festival day while it is still daylight to [potentially] permit it, and it fell into a pit on the Festival day, and he is afraid that it might die there.

Introduction A first-born animal may not be eaten until it becomes blemished (Deuteronomy 15:21-22). Experts would determine whether an animal was blemished. Our mishnah deals with a first-born that fell into a pit on Yom Tov and may have been blemished there. The debate is over whether they may send down an expert to see if the animal had become blemished and if so, bring it up and slaughter it on Yom Tov.

ירד מומחה – who is a specialist to distinguish between a fixed blemish and a passing blemish.

A first-born beast that fell into a pit:
Rabbi Judah says: let an expert go down and inspect [it]. If it had a blemish he may bring it up and slaughter it, but if not, he may not slaughter it. Rabbi Judah allows the expert to go down into the pit to examine the animal. If the animal is blemished they may bring the animal out of the pit and slaughter it on Yom Tov. But if there is no blemish he may not slaughter it nor may he even bring it up from the pit.

ויראה – a blemish that it had [acquired] yesterday, if it is a fixed blemish.

Rabbi Shimon says: any animals whose blemish was not observed on the day before the Yom Tov, it is not prepared (. Rabbi Shimon holds that even if the animal should be found to have a blemish, they still may not bring it out of the pit and slaughter it on Yom Tov because when Yom Tov began that animal could not be slaughtered. The animal was not “mukhan”, or ready. Mukhan is the opposite muktzeh. The animal is muktzeh because when Yom Tov began it could not be used for food because it was still a first-born on whom no blemish had been found.

יעלה וישחט – because there is no prohibition for use or handling [on Yom Tov] (or set aside in a shed for a sacrifice), and from yesterday, his mind was upon it.

ואם לאו לא ישחוט – this is how it should be understood, and if not, that this blemish that was upon it from the eve of the Festival was not a fixed [blemish] but rather on that day, it became a fixed blemish, even if it had passed and he brought it up, he should not slaughter it. And we don’t say that from yesterday, his mind was upon it and since it now has a fixed blemish, let’s slaughter it, for since yesterday’s blemish was not fixed, it is set aside in consequence of its being ritually forbidden.

אין זה מן המוכן – not because it was forbidden for use or handling did he prohibit it, for Rabbi Shimon does not have [the concept of] Muktzeh/something set aside, but because he permits its use on the Festival day, it is like he repairs it, and it appears that he made a legal decision that was decreed upon it because of it is forbidden by the Rabbis as being out of harmony with the celebration of the day. And this is how it should be state: His making it permissible is not a permit, and it is not prepared to be acceptable.