QUESTION: The Beraisa (89b) states that when one pays for a share in someone else's Korban Pesach, the money which the seller receives becomes Chulin. The Gemara explores the reason for this. Logically, the money should remain Hekdesh, because the buyer designated the money to be used for the Korban Pesach and thereby made it Hekdesh, and he gave the money in exchange for a share in another person's Korban Pesach, which obviously was Hekdesh. There is no object of Chulin in the transaction onto which the Kedushah of the money can be transferred.
Abaye explains that the Beraisa follows the opinion of Rebbi according to Rebbi Oshiyah's understanding, who maintains that when a person sanctifies the Korban Pesach, he does not sanctify the entire animal, but he leaves part of it as Chulin. It is that part of the animal onto which the Kedushah of the money is transferred.
The Gemara quotes the Mishnah in Temurah that says that the prohibition of an Esnan of a Zonah takes effect on any object of Hekdesh that is given to the Zonah as her wage. Rebbi Oshiyah says that the Mishnah there is in accordance with the view of Rebbi, who maintains that when a person sanctifies his Korban Pesach, he leaves over a portion which is not sanctified. It is that portion which, when given to a Zonah, becomes prohibited. Abaye says that if not for Rebbi Oshiyah's explanation of the Mishnah in Temurah in accordance with the view of Rebbi (wherein Rebbi Oshiyah shows that Rebbi maintains that part of the Korban Pesach is not sanctified), Abaye would have explained that the Mishnah there is in accordance with the view of Rebbi Yosi ha'Glili (who says that an object of Kodshim Kalim belongs to the owner and not to Shamayim).
According to the explanation that Abaye would have offered, the Beraisa here would have followed the view of Rebbi but for a different reason. The money becomes Chulin not because the animal is not fully sanctified, but because the buyer originally stipulated that if the animal in which he acquires a share is already sanctified, then the money should not be sanctified. The Beraisa here cannot be in accordance with Rebbi Yosi ha'Glili, because the Beraisa also says that a Korban Shelamim cannot be sold, but Rebbi Yosi ha'Glili maintains that a Shelamim can be sold (it is "Mamon Ba'alim," the property of its owner).
According to the Beraisa itself that says that a Korban Shelamim cannot be sold, how can a share in a Korban Pesach be sold? The Beraisa maintains that one cannot sell or conduct any transaction with a Korban Shelamim, because it is not considered to belong to him (but rather to Shamayim). Even if the "sale" is conducted by having the owner give to the "buyer" a share in the Korban as a gift, and the "buyer" in return gives money to the owner as a gift, so that no actual sale is conducted, why is the owner of the Korban Pesach entitled to give a share of the Korban as a gift? He is not considered the owner of the animal anymore!
If, for some reason, the Korban Pesach differs from a Shelamim and has a special allowance to be bought and sold, then it should also become forbidden when given to a Zonah as payment even when no portion of the animal was left over as Chulin.
ANSWER: TOSFOS (89b, DH v'Zeh) answers that the Gemara (89a) derives from a verse that a person may designate others ("Menuyim") to eat from his Korban Pesach until it is slaughtered. The Gemara implies that the designating of others as owners of a Korban Pesach ("Minuy") does not need to be done at the time one sanctifies the animal. The Torah allows it to be done even after the animal has been sanctified.
Tosfos explains that the mechanism behind this Gezeiras ha'Kasuv is that at the moment that a person sanctifies an animal as a Korban Pesach, he has in mind that whoever will join him later will be included among the owners of this Korban, and thus he sanctifies it for them as well at that moment. It becomes the Korban of the others who join later, retroactively from the time that he sanctified the animal.
If one gives a share in the animal to a Zonah, he does not give it in exchange for services rendered, but rather he sanctifies it for the purpose of her fulfillment of the Mitzvah of Korban Pesach.
Similarly, when one appoints others to be included in his Korban Pesach, he gives them their shares as a gift. Any money that they pay in return for their shares is not money of a "Kinyan" (acquisition), because they already own their shares in the Korban from the time that the owner sanctified it. Rather, the money they give him is a gift.
(If, as Tosfos writes, there is a logical basis for why the owner can transfer shares of his Korban Pesach to others (i.e., because he had those people in mind at the moment he sanctified the Korban), then why does Tosfos call it a "Gezeiras ha'Kasuv"? The answer is that without a Gezeiras ha'Kasuv in the Torah, such a condition at the moment of sanctification of the animal would not work, because the Halachah is "Ein Bereirah." Even according to the opinion that maintains "Yesh Bereirah," the Gezeiras ha'Kasuv is necessary, because without it we would not know that every person who sanctifies an animal as a Korban Pesach has this condition in mind.)