Temurah Chapter 6, Mishnah 3. In the first mishnah of the chapter we listed nine animals whose life history has rendered them unfit to serve as an offering, among them esnan zonah and mechir kelev. Esnan zonah is the payment given to a harlot, and mechir kelev is the item given in exchange for the purchase of a dog - and both of them, when the item in question is an animal fit for the altar (a cow, a goat or a sheep), are disqualified from being brought as an offering.
The source of the law is the verse: "Lo savi esnan zonah u'mechir kelev beis Hashem Elokecha l'chol neder, ki so'avas Hashem Elokecha gam sheneihem" - you may not bring into the House of Hashem your God, for any vow, either the payment given to a harlot or the item given in exchange for a dog, for both of them are an abomination before Hashem your God.
On the basis of that same verse, the previous mishnah already discussed the details of esnan zonah, while in our mishnah and the one that follows we deal with mechir kelev and the matters connected to it.
"Eizehu mechir kelev?":
The simple case is "Ha'omer la'chaveiro: hei'lach taleh zeh tachas kelev zeh" - Reuven says to Shimon: take my lamb in exchange for your dog, you receive the lamb and I receive the dog. The lamb that came into Shimon's hands is unfit to serve as an offering, because it is mechir kelev, the item given in exchange for the dog.
The second case - two partners who divided their property:
Here we are dealing with an entirely different case: Reuven and Shimon jointly own nineteen sheep and one dog, and they divide the animals between them, ten animals each - one takes ten sheep, and the other takes nine sheep and the dog. The law is that the one who took the ten sheep may not use any of them as an offering, whereas the one who took the nine sheep and the dog may bring any of them as an offering. What is the difference? The ten sheep were given in exchange for the dog, since they are what was traded for it.
It is worth emphasizing which case the mishnah is not discussing: it is not a situation in which all the sheep are worth six, and among them is an especially valuable ram worth seven, and the dog too is worth seven - so that Reuven takes the dog, Shimon takes the ram, and the remaining eighteen sheep are divided equally between them. In such a case it is obvious that the mechir kelev is that ram alone, and the rest of the sheep that were divided are permitted without any restriction.
The case the mishnah is discussing is this: ten of the sheep are worth six each, nine of them are worth five each, and the dog is worth fifteen:
Reuven: takes ten sheep worth six each - sixty in all.
Shimon: takes nine sheep worth five each - forty-five, and in addition the dog, worth fifteen - sixty in all.
It turns out that both of them received equal value, except that the dog, worth fifteen, was in effect paid for in part by each and every one of the more valuable sheep that the other partner received. Therefore all ten of those sheep are forbidden, since each one of them formed part of the exchange for that dog.
And this is the language of the mishnah: "V'chein shnei shutafin she'chalku, echad natal asarah v'echad natal tishah v'chelev":
"Shekeneged hakelev asurim" - the ten sheep that stood on the other side of the exchange, opposite the dog; since each one of them served as partial payment for it, they are all forbidden.
"She'im hakelev mutar" - the nine sheep that came together with the dog were not exchanged for it, and therefore they may be used for offerings.
The hire of a dog and the price of a harlot:
The verse forbids bringing the hire of a harlot and the price of a dog. What is the law in the reverse cases - the hire of a dog and the price of a harlot?
The hire of a dog: a person who wishes to rent his friend's dog in order to sin with it, and pays for this with a lamb.
The price of a harlot: a person who is not satisfied with hiring the woman's services, but purchases her as a Canaanite maidservant, and pays for this with a lamb.
Are these lambs forbidden? The Mishnah rules that they are not. Logic might have inclined us to say otherwise, but the Sages have a derivation teaching that the prohibition was stated specifically regarding the hire of a harlot and the price of a dog.
The verse concludes: "ki so'avas Hashem Elokecha gam sheneihem" - "for both of them are an abomination to Hashem your God". What do the words "gam sheneihem" come to add? After all, the number two is known even without them. Rather, the word "sheneihem" is expounded as a compound word - "shenayim" (two) and "hem" (they) - and from it two derivations are learned.
The language of the Mishnah: "Esnan kelev umechir zonah - harei elu mutarim" - those animals are fit for use, "sheneihem" - two and not four. Only the hire of the harlot and the price of the dog were forbidden, while the payment given for the dog and the price given for the harlot are not forbidden.
The Mishnah further rules: "Veladoseihem mutarim" - an animal that was forbidden as the hire of a harlot or the price of a dog, its offspring is permitted. This teaching too is derived from the word "sheneihem": "they and not their offspring" - they themselves, that very ewe with which the hire or the price was made, are forbidden, but not their offspring.
In summary: in this Mishnah we learned what constitutes the price of a dog - both in the simple case of "here is this lamb in exchange for this dog", and in the case of two partners who divided their property, in which the animals set opposite the dog are forbidden while those that came along with the dog are permitted. We also learned that the hire of a dog and the price of a harlot are permitted, from the derivation "two and not four", and that the offspring of the forbidden animals are permitted, from the derivation "they and not their offspring".