1) MAY ONE RETURN A LOST OBJECT TO A PERSON WHO IS PROHIBITED FROM RECEIVING BENEFIT FROM HIM?
The Mishnah (33a) states that when one makes a Neder not to give pleasure to his friend, he is permitted to return his friend's lost object to him. The Gemara here explains that he is not giving his friend any new pleasure. Rather, he is merely returning his friend's own object to him.
However, the Gemara in Kesuvos (108a) explains that he is permitted to return his friend's lost object because he is doing a Mitzvah.
Why does the Gemara here give a different reason for why it is permitted?
A practical difference between the two reasons is whether or not he may give Tzedakah to his friend. According to the Gemara in Kesuvos, he should be permitted to give Tzedakah to his friend, since Tzedakah is also a Mitzvah. According to the Gemara here, he should not be permitted to give Tzedakah to his friend, because the money of Tzedakah is something new that never belonged to him.
ANSWER: When one returns a lost object to his friend, his friend receives pleasure from two possible sources. He experiences the pleasure of the favor that his friend did for him by returning his object. He also experiences pleasure from having his item back in his possession (regardless of how it got there). The reason the Gemara here gives and the reason the Gemara in Kesuvos gives are necessary in order to explain why each of these sources of pleasure do not make it prohibited to return his friend's lost object. The first source of pleasure, that his friend did him a favor, is not prohibited because it is not considered as though he is receiving pleasure from his friend. Rather, he is receiving pleasure from the Mitzvah (that his friend is obligated to fulfill). Still, however, his friend's fulfillment of the Mitzvah provides him with another pleasure -- having his item back. The Gemara here says that since it was his item to begin with, the return of the item to his possession is not considered a pleasure that is prohibited.
The TALMIDEI RABEINU YONAH, quoted by the SHITAH MEKUBETZES in Kesuvos, explain that when one returns a lost object in order to fulfill the Mitzvah of "Hashavas Aveidah" the pleasure the recipient experiences does not come from the finder but rather from the Mitzvah. (The one who returns the item does not do so out of his desire to give pleasure to his friend. Rather, he is acting as a Shali'ach, so to speak, in carrying out Hash-m's will, as HAGA'ON RAV YISRAEL ZEV GUSTMAN zt'l explains in KUNTRESEI SHI'URIM.)
The Talmidei Rabeinu Yonah explain that although giving Tzedakah is a Mitzvah, one may not give Tzedakah to the person who is prohibited from receiving pleasure from him since he can give his Tzedakah money to someone else and is not required to give his Tzedakah to one particular person. (Giving Tzedakah to any particular person is a result of his personal decision and is not a result of the directive of the Mitzvah.) Normally, however, one is permitted to give money to someone who is prohibited to receive benefit from him when he thereby performs a Mitzvah.
In contrast, the RITVA in Kesuvos explains that one may never give money to someone who is prohibited to receive benefit from him, even when there is no other choice but to give it to that person, since he is increasing his friend's assets by giving him money. Hence, when he gives his friend Tzedakah, his friend receives pleasure from the giver's money, even though the money was given only because of the Mitzvah of Tzedakah. Therefore, the benefactor is prohibited to give him Tzedakah even though he thereby fulfills a Mitzvah.
The Gemara here in Nedarim explains that the finder is not giving his friend anything new; he is merely returning his friend's own object. (The RAN here also emphasizes that he is merely "chasing away a lion" from his friend's property.) However, his friend still receives some degree of pleasure from the return of the lost object. Therefore, the Gemara in Kesuvos explains that the finder may perform this service because he is not doing it for his friend's benefit but in order to follow Hash-m's command to do the Mitzvah, and therefore his service is not an act of giving pleasure to his friend and is permitted.
(b) The RASHBA here explains that when one makes a Neder not to give pleasure to (or receive pleasure from) his friend, he does not intend to prohibit pleasure from acts of Mitzvos which his friend performs. However, his intention does not extend to giving something new to his friend, but rather only to situations in which he gives nothing new to his friend (for example, visiting his friend who is sick). Giving money (such as Tzedakah) to his friend is prohibited by his Neder, because his friend will have pleasure from the money after the Mitzvah is completed, and he thus will derive pleasure from more than just the act of the Mitzvah. Similarly, when one returns a lost object to his friend, his friend not only derives pleasure from the act of the Mitzvah but he continues to derive pleasure from having his item back. Although at the moment the object is returned the pleasure he has is not prohibited because that pleasure is a result of the Mitzvah of Hashavas Aveidah, after the Mitzvah is completed the owner of the item continues to experience the pleasure of having his item back. Hence, the Gemara here does not give the same reason ("Mitzvah Ka Avid") which it gives in Kesuvos because it disagrees with that reason and maintains that it does not suffice. The Gemara here therefore says that the Hana'ah in this case is not prohibited because he is only giving back his friend's own item to him.
(c) The RAN later (39b) writes that the fact that one is doing a Mitzvah suffices to make the Hana'ah permitted only when the Hana'ah comes automatically as a result of the Mitzvah (such as visiting the sick). When the Hana'ah takes the form of pleasure given directly from the person to his friend, it is prohibited even though he is doing a Mitzvah. This explains why the Gemara here is not satisfied with the reason of "Mitzvah Ka Avid" in order to explain why one is permitted to return the lost object, since in this case he actually gives his friend direct Hana'ah when he returns the object.
2) DOES A GUEST OWN THE FOOD WHICH HIS HOST GIVES HIM
QUESTION: The Gemara discusses the case of a person who declares to his friend, "My loaf of bread is [prohibited] upon you," and then he gives the loaf to his friend as a gift. The Gemara asks whether his friend may eat the loaf because the owner said only that "my loaf" is prohibited, but now it is no longer his loaf, or whether his friend remains prohibited from eating the loaf because the wording of the Neder implies that the loaf is prohibited "upon you" regardless of who owns it.
The Gemara asks that if the loaf is permitted when he gives it to his friend, in what way did he intend to prohibit it with his Neder? The Gemara answers that the owner meant to prohibit his friend from eating the loaf in the event that he invites his friend to his home to eat with him.
The Gemara seems to conclude that when a host serves a loaf of bread to a guest, the guest is not considered to be the owner of the bread. Consequently, if a person makes a Neder to prohibit his friend from eating his bread and then he invites his friend to his home and serves him bread, the guest is not allowed to eat the bread since the host is still considered its owner.
If a guest indeed does not obtain full ownership of his portion of food before he eats it, then perhaps he may do with the food only what the owner (the host) intended. He has permission only to eat it, and he may not give it to anyone else. Is this indeed the law?
(a) The RAN and RITVA indeed learn that this is the Gemara's conclusion. Accordingly, a guest does not acquire ownership of the food served to him, and he may not do with the food anything he pleases.
(b) The MEFARESH infers the opposite point from the Gemara. He understands that the Gemara means that if a host invites a guest to eat with him and serves him food, and then he declares, "My food is prohibited to you," the food he gave the guest does not become Asur since the guest already acquired it and it no longer belongs to the host.
The ROSH also understands the Gemara differently. The Rosh explains that the Gemara means that one is prohibited to invite his friend to come eat the bread, since his friend will have pleasure from the invitation even before he comes and eats. According to the Rosh, it is possible that the guest fully acquires his portion of food when he picks it up to eat it since the pleasure which the host prohibited is the pleasure of the invitation and not the actual eating.
The NIMUKEI YOSEF understands that the Gemara's question involves merely the intention of the person who makes the Neder. The Gemara is not discussing the technical question of what belongs to him. Even if the guest who eats the bread acquires it when he eats it, the intention of the owner was that the other person should not benefit from the bread by acquiring it by eating it "at my house." The owner of the bread allows, however, for the person to benefit from the bread if he receives it as a gift outside of the house and eats it later.
HALACHAH: The MAHARIT (Teshuvos #150) cites the Mishnah in Demai (7:1) which states that when one is a guest at the table of someone whom he does not trust to have separated Terumos and Ma'aseros, he is permitted to separate the Terumos and Ma'aseros himself from the food that is served to him. This act is not considered stealing even though he is giving away part of the food that was given to him and is not eating it. The Yerushalmi in Demai explains that it may be assumed that the host wants his guest to enjoy himself and therefore permits him to separate Terumos and Ma'aseros from the food. The Maharit writes that the Mishnah there does not provide proof that the guest owns the food that is given to him and that he may do whatever he wants with it. There are grounds to say that since the host invited the guest to eat with him, the host granted him permission to do anything with the food that would enable him to eat it (such as to separate Terumos and Ma'aseros from it), but he does not give him permission to use it for other purposes.
The Maharit quotes the Gemara in Bava Metzia (87b) which concludes that a worker in a field has the right to eat fruit from the field in which he works, but he is not permitted to give fruit from the field to his family. The Maharit attempts to prove from there that a guest does not have permission to give his portion to others.
On the other hand, the Maharit quotes the RI'AZ (Kidushin, end of second Perek) who writes that if one of the guests gives his portion to a woman for the sake of marrying her, the Kidushin does take effect. It is assumed that the host gives his guests full ownership over their food, and thus they may do whatever they want with their food. The Maharit remains in doubt about this issue.
The REMA (EH 28:17) rules that when a guest takes the food given to him by his host and gives it to a woman for Kidushin, the Kidushin is valid. The Rema apparently maintains that a guest does own the food that his host gives to him.
However, the SHULCHAN ARUCH (OC 170:19) rules that a guest is not permitted to take the food that the host gives him and give it to the host's children or servants without permission of the host (based on the Gemara in Chulin 94a). The TAZ (EH 28:34) infers from this ruling that the guest does not own the food. He writes that when the Rema rules that the Kidushin is valid, it is only a Safek Kidushin and thus the woman needs a Get. However, the simple meaning of the Gemara in Chulin is that the guest may do with the food whatever he wants except give it to the members of the household, because doing so may threaten the harmony in the home (see the incident recorded by the Gemara, Chulin 94a). This seems to be the intention of the BEIS SHMUEL (EH 28:46) who writes that the guest does own the food but he is not permitted to give it to the host's children for a different reason. (See also MISHNAH BERURAH OC 170:40.)
It is interesting to note that none of the Poskim quote the Gemara here in Nedarim as proof that the guest does not own the food that his host gives him. RAV SHLOMO KLUGER (in NIDREI ZERIZIM) writes that no Posek quotes the Gemara here because the Halachos of Kidushin and other laws cannot be learned from the Halachos of Nedarim. Nedarim depend on the way people speak, and thus the law with regard to Nedarim cannot establish the legal status in the case of other categories of Halachah. A person says "my loaf" even when he invites someone to come eat with him and fully gives the loaf to the guest, and he does not actually mean that he retains ownership of the loaf when he gives it to the guest.
Whether or not a guest owns the food that his host gives him is relevant in practice on Pesach night. The Gemara in Pesachim (38a) teaches that the Matzah used for the Mitzvah must be owned by Jews ("Lachem"). Accordingly, the SEFAS EMES (Sukah 35a) infers that one must be the owner of the Matzah in order to fulfill the Mitzvah of eating Matzah on Pesach night. He points out that according to this, if one has guests at his Seder, he should have them acquire the Matzos before they eat them. He admits that the custom is not to do so, and he explains that since it is necessary to acquire the Matzos it is assumed that the host gave them the Matzos in a manner in which they acquire it fully before they eat it. (See Insights to Pesachim 38:2.)