Arachin, Chapter 8, Mishnah 7. The previous mishnah closed with the ruling that a person may declare a cherem upon his own kodashim. Our mishnah picks up that ruling and works it out in detail: if the animal itself is already committed to the mizbeach, what exactly is it that becomes cherem, and how is that value calculated? The mishnah then extends the discussion to a bechor, and finally brings Rabbi Yishmael's derivation of the whole principle from two verses that appear to contradict one another.
Declaring cherem on one's own korbanot
The opening words are "Macharim adam et kodashav": animals a person has already consecrated as korbanot are nevertheless eligible to be placed under a cherem for the benefit of the Beit Hamikdash's funds. The mishnah draws no line between the grades of sanctity involved, "bein kadshei kodashim", offerings of the highest grade, "bein kodashim kalim", those of the lighter grade. Either may be made the subject of such a declaration.
A difficulty presents itself at once. The beast cannot literally be converted into cherem property. Its standing as a korban was fixed the moment it was consecrated, its sanctity binds it to the mizbeach, and nothing declared afterward can dislodge that. On what, then, does the declaration take hold? The mishnah's response is that it fastens onto a sum of money bound up with the animal, and the size of that sum depends on the sort of commitment its owner had originally accepted.
A neder: paying the animal's full price
"Im neder, noten et hadamim": where the korban originated in a neder, the owner having first undertaken by vow to bring an offering and only afterward picked out this particular beast to discharge that undertaking, he hands over its full price in money. The reason is that his commitment was never tied to this specific animal. Had it strayed or been taken from him, the vow to Hashem would still stand and he would have to purchase a replacement. The beast is worth to him exactly what a suitable substitute would fetch, and it is that objective price he pays in.
A nedava: paying the value of the benefit
"Im nedava, noten et tovato": in a nedava no earlier vow ever bound the owner; he simply took this animal and consecrated it. His entire commitment therefore lives in this one beast. Were it to be lost or stolen, nothing at all would remain outstanding, since no independent obligation was ever created. There is accordingly no objective price to demand of him. What is collected is his tovah, the worth of the advantage the animal affords him: what a person would part with for the privilege of presenting Hashem a gift he was never required to bring. That figure, understandably, comes out much smaller.
The mishnah lays out the reckoning. A man declares "shor zeh olah", this ox shall be an olah, and afterward places the ox under cherem. The appraisers now inquire: "kama adam rotzeh liten", what would someone be ready to spend, "beshor zeh", on this very ox, "leha'aloto olah she'eino rashai", in order to bring it up as an olah he was never required to offer, purely as a present to Hashem? Whatever that figure comes to fixes the tovah, and that is the amount transferred over to the cherem.
Declaring cherem on a bechor
Next the mishnah takes up the firstborn animal: "habechor, bein tamim bein ba'al mum", whether it is sound and thus fit for the mizbeach or carries a blemish that disqualifies it from ever being brought, "macharimin oto", a cherem may be placed upon it. The case under discussion is a bechor still held by the owner of the mother animal, before it has been transferred to any kohen.
At first glance this is puzzling. The bechor is not really the owner's property at all; it belongs to the kohanim. What is there for him to declare cherem? The answer is that the owner does hold something of value: the right to decide which kohen will receive the animal. That choice can win him the goodwill of a particular kohen, and because it is worth something to him, his cherem takes effect upon it.
"Keitzad podin oto?" How is it redeemed, that is, how do we attach a price to what this bechor is worth to the man holding it? The appraisers, "hapodin", estimate "kama adam rotzeh liten", what a second party would be willing to hand this owner, "babechor zeh", regarding this bechor, "liteno leven bito", that he give it to that party's grandson who serves as a kohen, "o leven achoto", or to his nephew, his sister's child, who is a kohen. Someone in possession of a bechor may lawfully accept a modest payment for steering the animal toward one particular kohen instead of another. That payment measures the cherem, and it is what the owner deposits with the Beit Hamikdash.
Rabbi Yishmael: reconciling two verses
The mishnah now supplies the scriptural basis for the arrangement just described: a bechor's monetary worth is open to cherem, while the beast itself is closed to any further act of consecration.
"Rabbi Yishmael omer", Rabbi Yishmael points to two verses that pull in opposite directions concerning the bechor. "Katuv echad omer takdish", one passage instructs us to sanctify it, which suggests there is indeed some act of sanctification left for a person to carry out. Yet "vechatuv echad omer al takdish", a second passage tells us not to sanctify it, since a bechor emerges from the womb already holy and requires no act of ours at all.
Neither verse can be taken at face value in isolation. "I efshar lomar takdish", the first cannot be a straightforward instruction to sanctify the bechor, "shekvar neemar al takdish", because the other passage has already informed us that no sanctification is called for. The reverse fails equally: "ve'i efshar lomar", we cannot read the second passage as an outright prohibition, "al takdish", against consecrating it, "shekvar neemar takdish", since the first has explicitly directed us to consecrate.
So how are the two verses to be understood together? "Emor me'atah: makdisho ata hekdesh ilui, ve'ein ata makdisho hekdesh mizbeach." Say from here that the verse commanding consecration refers to hekdesh ilui, a consecration of value: one may consecrate the worth of the bechor, or declare that worth cherem. The verse forbidding consecration refers to hekdesh mizbeach, sanctity of the altar: one may not consecrate a bechor so as to give it a new korban status.
The two verses thus divide neatly. Where the Torah says you may consecrate, it means the value, the sum that the bechor is worth to its holder, and on that the cherem takes effect. Where the Torah says you may not consecrate, it means altar sanctity: a bechor cannot be turned into a different kind of korban. Once an animal is a bechor, it remains a bechor forever.