Gemara

9b (Mishnah): If it gave birth to a male and female, he designates a Seh...

Question: Since he keeps it, why must he designate it?

Answer: This is to permit the Peter Chamor.

Inference: Before designation, the Peter Chamor is Asur b'Hana'ah.

(Beraisa - R. Yehudah): One may not benefit from a Peter Chamor;

R. Shimon permits this.

Question: What is R. Yehudah's reason?

Answer #1 (Ula): We never find something that can be (fully) permitted (for less than its value) only with a Seh, yet one may benefit from it.

(Mishnah - R. Yehudah): If Reuven was Mekadesh Leah with Ma'aser Sheni b'Mezid, she is Mekudeshes.

Likewise, if he was Mekadesh her with a Peter Chamor, she is Mekudeshes.

She knows that Peter Chamor is Asur b'Hana'ah. She accepted it intending to redeem it for a Seh. She is Mekudeshes with the excess value of the donkey above the amount she must spend to redeem it.

(Ula): R. Shimon holds that we never find something that is forbidden, yet what was used to redeem is permitted.

10b (Mishnah): If Reuven was Mekadesh Leah with a Peter Chamor, she is not Mekudeshes.

Question: The Mishnah is not like R. Yehudah, nor like R. Shimon!

R. Shimon permits benefit from a Peter Chamor. He holds that she would be Mekudeshes!

R. Yehudah holds that she is Mekudeshes with the value of the donkey less the cost to redeem it!

Answer (Rabah bar Avuha): It is like R. Yehudah. The case is, the Peter Chamor is worth only one Shekel. (It has no net value, for) R. Yehudah holds like his son;

1. (Beraisa): It says "Tifdeh" twice teaches that a Peter Chamor may be redeemed immediately, and it may be redeemed for (a Seh worth) any amount;

2. R. Yosi bar Yehudah says, the (Seh for) redemption must be (worth) at least one Shekel.

1. 11a (Ravina): The general rule is, when R. Yehudah and R. Shimon argue, the Halachah follows R. Yehudah. Also, our Stam Mishnah is like R. Yehudah!

Rishonim

Rambam (Hilchos Bikurim 12:4): A Peter Chamor is Asur b'Hana'ah until it is redeemed. If one sold it before it was redeemed, the money is forbidden.

Rebuttal (Ra'avad): If the money is forbidden, why doesn't he give it to a Kohen, and the Kohen can benefit from it? A Peter Chamor can be redeemed for its value. Is it an Isur Hana'ah that its Tofes (forbids) the money used to redeem it?!

Kesef Mishneh: What is his question? The Rambam says just like the Gemara! Perhaps he questions the Gemara.

Rosh (1:11): The Halachah follows R. Yehudah against R. Shimon. Also, the Stam Mishnah is like R. Yehudah. We forbid Hana'ah of its body and its value. Hana'ah of its value is selling it to one who does not know that it is a Peter Chamor, and selling it for the full value. If he tells the buyer that it is a Peter Chamor, and he pays less because he knows that he must redeem it, this is permitted. Hana'ah of its body is working with it, or shearing it. If its hair was tangled and harming it, or it had a wound where there is hair, he shears it and buries the hair. Shearing is forbidden only due to Hana'ah, since the donkey has only Kedushas Damim (its value). It is lower than other cases of Kedushas Damim, for which Chachamim decreed to forbid shearing and working, for there, the redemption becomes Kodesh. The redemption of a Peter Chamor is Chulin.

Poskim

Shulchan Aruch (YD 321:8): A Peter Chamor is Asur b'Hana'ah until it is redeemed. If one sold it before redemption, the money is forbidden.

Shach (5): Even what it is worth more than the Seh is forbidden. Since he did not tell the buyer that it is a Peter Chamor, the buyer will not redeem it, and it is totally forbidden.

Question: Here, the Shulchan Aruch rules like the Rambam, that a Peter Chamor is Tofes (forbids) the money given to redeem it. In EH (28:22), the Shulchan Aruch says that if one sold Isurei Hana'ah and was Mekadesh with the money, she is Mekudeshes. The Pidyon money of the Peter Chamor should be forbidden. Why is she Mekudeshes?

Answer (Noda bi'Yehudah 1 YD 84, cited in Pischei Teshuvah 1): The Poskim do not argue about whether Pidyon Peter Chamor is permitted or forbidden. The Tana'im argue about this! The Halachah follows R. Yehudah, that it is forbidden. The Rosh permits only if he told the buyer that it is a Peter Chamor. If he did not, all the Poskim forbid. This question can be asked against the Rambam, who explicitly says that Kidushin with Pidyon Peter Chamor takes effect. A Mishnah (Kidushin 56b) says so, and the Gemara says that this is like R. Yehudah, and it can even be like all the Tana'im! The Ra'avad asked why the Pidyon is forbidden. This is a difficult question, for we hold that only idolatry and Shevi'is are Tofes their Pidyon! I suggest that this rule applies only to Isurei Hana'ah that cannot be permitted, and similarly their Pidyon cannot be permitted. Peter Chamor is different, for it has a remedy. After Pidyon, one may benefit from it. The money is no more stringent than the Peter Chamor itself. One can redeem the donkey even after it was sold and it is not his. Rav Shizbi (11a) taught that if one redeemed another's Peter Chamor, it is redeemed. The Torah forbade Hana'ah from Peter Chamor only to prevent a loss to the Kohen (lest Yisre'elim not redeem it). If one did not redeem it, we do Arifah (break its neck) and it is Asur b'Hana'ah. The Gemara explained according to R. Yehudah that since he caused a loss to the Kohen, he loses. Why is a reason needed? It was Asur even while it was alive! Rather, the Isur while it was alive is only to prevent a loss to the Kohen. The sale money is forbidden, for he can still redeem it. Other Isurei Hana'ah cannot be permitted, therefore the Torah did not forbid the money paid for them. Therefore, if one gives the value to a Kohen, the money paid for the Peter Chamor is permitted.

R. Akiva Eiger: The Gemara connotes that if the Mishnah is like R. Yehudah, the law applies even while it is alive. Why is it Tofes the money paid for it (since the donkey is still forbidden)? Hekdesh is redeemed whether one says that he gives the money for Pidyon, or to buy it. Pidyon Peter Chamor should be the same. A sale should permit it, just like Pidyon. We must say that the Rambam discusses when we cannot say that the sale was Pidyon, e.g. it was sold for less than its value. Also, we can say that a sale of Hekdesh is Pidyon only when the buyer knew that it is Hekdesh. We can say that the Mishnah in Kidushin discusses when it was sold for its value, and the donkey and the money paid for it are permitted due to Pidyon. However, the Gemara concluded that the Mishnah discusses after Arifah. Then it cannot be redeemed, so the money should be forbidden! This strengthens the Ra'avad's question. We must say that when Pidyon cannot permit the Isur, it cannot forbid the Pidyon money. A support for this is that the Rishonim wrote that the Heter of throwing the money of the Isur to the Dead Sea applies only to idolatry, which is Tofes its Pidyon. Money does not permit idolatry. Even so, since idolatry is Tofes what is paid for it, this is like Pidyon, so regarding a mixture we are lenient to throw some to the sea and this is like Pidyon. Here we say conversely. Since Pidyon permits the donkey, a Peter Chamor forbids the money paid for it. After Arifah, Pidyon cannot permit it, therefore it does not forbid money paid for it, like other Isurim.

Rema: This is only if he sold it Stam. If he told the buyer that it is a Peter Chamor, and he sells its excess value over the value of a Seh to redeem it, the money is permitted. L'Chatchilah one may sell it in this way.

Beis Yosef (DH Peter): The Gemara connotes that R. Shimon and Chachamim argue about Hana'ah from the value of a Peter Chamor, and from it itself. The Rambam and Rosh rule like the first Tana. The Rosh connotes that if the buyer knows that it is a Peter Chamor, even if he gave the full value, it is permitted. Since he knows that he must redeem it, we must say that to him it is worth all the money he pays, plus the value of the Seh needed to redeem it. Alternatively, he gives a gift to the seller. (The extra amount) is not part of the price of the Peter Chamor, for he must redeem it. The Tur connotes that even if the buyer knows that it is a Peter Chamor, one may not sell it for the full value. The Rambam did not distinguish.

Taz (8): The Rema is like the Tur. If it depends on not paying the full value, why did the Tur mention telling the buyer that it is a Peter Chamor? Even if he told him, he may not take the full value! We can say that he discusses the normal case. When he tells the buyer, the buyer does not pay the full value. If one pays the full value even though he knows that it is a Peter Chamor, the seller transgresses. The Beis Yosef explained that the Rosh holds that it depends only on informing him. If he told him, he may receive the entire value. I say that surely the buyer and seller intended for Heter, and not Isur. Whatever he agreed to give was for the excess, even if it is not worth so much in the market. We find that when a Ba'al ha'Bayis sells his Kelim, there is no Ona'ah (Isur of overcharging), for (it is known that) he sells his Kelim for a high price. It is forbidden only if the buyer specifies that he pays for the entire value, and he will not give a Seh to a Kohen.

Gra (20): The Rosh learns from the Gemara, which said that a woman is Mekudeshes with the excess value of the Peter Chamor above the Seh needed to redeem it.

Rema (ibid.): If one did not redeem it and gave the Peter Chamor itself to a Kohen, he may not use it until he redeems it with a Seh and he keeps the Seh himself, or he does Arifah and buries it. Kohanim are suspected about this, therefore one may not give a Peter Chamor to a Kohen unless he redeems it or does Arifah in front of him.

Shach (6): Kohanim are suspected because they reason 'why must I redeem it? In any case I keep the Seh!'