Gemara

(Mishnah - R. Tarfon): If one of the twins (it is a Safek which is the Bechor), the owner and the Kohen share the other animal.

R. Akiva says, ha'Motzi me'Chavero Alav ha'Re'ayah.

(Rava): The case is, a Kohen takes care of the Yisrael's animals on the Yisrael's premises. R. Tarfon holds that the Yisrael transfers ownership of his land to the Kohen, so that the Kohen will acquire Bechoros when they are born, for he wants the Mitzvah;

R. Akiva holds that the Yisrael is not Makneh, for he does not want to lose (Safek Bechoros).

Bava Metzi'a 29b (Mishnah): If one found Seforim, he reads in them once every 30 days;

He may not learn in it something he has not learned before, and two people may not read together.

(Shmuel): If one found Tefilin in the market, he estimates their value and may wear them immediately.

Question (Ravina - Mishnah): If one found Seforim, he reads in them once every 30 days.

The Mishnah does not say to estimate them and take them!

Answer (Abaye): Tefilin are readily available to buy. It is not easy to buy Seforim.

(Beraisa): If Reuven borrowed Shimon's Sefer Torah, he may not lend it to Levi.

Question: Why was this taught about a Sefer Torah? Reish Lakish taught that one who borrowed any object may not lend it to another person!

Answer: One might have thought that a Sefer Torah is different, for one is happy that people do a Mitzvah with his property. The Beraisa teaches that this is not so.

(Beraisa): If Shimon deposited his Sefer Torah with Reuven, Reuven rolls it every 12 months, opens it and reads in it. If he opened it for his own needs, he may not read in it.

Pesachim 4b - Question: If one rented out his house on the 14th on the pretense that it was checked, and it was found to be unchecked, is this a 'Mekach Ta'os' (a mistaken rental, and the tenant may retract)?

Answer (Abaye): The rental is valid where people themselves check for Chametz, and even in a place where they hire others to check, for people like to do Mitzvos with their own money.

Rishonim

Rosh (Teshuvah 93:3): Ploni had Seforim and did not want to lend them. A judge fined him 10 gold pieces every day, because there was Bitul Torah in the city due to lack of Seforim. The judge acted properly. However, experts must estimate the value of the Sefer, and if it depreciates, the borrower must pay according to their estimation. He must give a security as great as the Sefer's appraised value.

Nimukei Yosef (Hilchos Ketanos 12a DH Talis, citing the Ritva): The custom is to bless on a borrowed Talis. However, Seforim are different. Perhaps one will read much in them, and they will tear from great use.

Nimukei Yosef (Bava Basra 44b DH Amar ha'Mechaber): The Ritva says that since we hold that Sho'el she'Lo mi'Da'as is a thief, one may not put on another's Tefilin or Talis without the owner's permission. My Rebbi says that a Mitzvah is different, for one is happy that people do a Mitzvah with his property. I disagree. The Gemara explicitly says that one who finds Tefilin may put them on only if he estimated their value and acquired. It is like other Aveidos, which one may not use. We do not permit due to the Mitzvah. One who borrowed a Sefer Torah may not lend it to another. One might have thought that a Sefer Torah is different; a Beraisa teaches otherwise.

Rebuttal (Gra OC 14:4 DH v'Hu): The Gemara permits one who found Tefilin to estimate their value and may wear them immediately (he buys them for himself). It does not forbid wearing them without doing so.

Rashi (29b DH ba'Tchilah and DH v'Lo): One who found Seforim may not learn in it something new, lest he leave it (open) in front of him for a long time. Two may not learn together, lest each pull it towards himself, and it will tear.

Terumas ha'Deshen (100): One is happy that people do a Mitzvah with his property, e.g. a Talis. Seforim are different, for they get dirty and ruined when one learns in them. The Gemara forbids learning in them Stam, i.e. even a little. This is because there no limit to how much one learns.

Mordechai (Bava Metzi'a 293): The Mishnah teaches about a Sefer deposited with an ignoramus. He may not open it to read for his needs. Rav Yehudai Gaon says that similarly, he may not copy even one letter from it, for it harms the deposit. If he deposited with a Chacham, l'Chatchilah he may read in it or copy from it if he does not have another such Sefer. The depositor knows that he will do so, and deposited with him with this intent. This is like one who deposits unwrapped coins. The Shomer may use them. Perhaps the reason is like the Midrash on Mishlei 6:30 (Yalkut Shimoni 939) expounds "Lo Yavuzu la'Ganav Ki Yignov" - we do not disgrace one who 'steals' Divrei Torah and copies them.

Poskim

Shulchan Aruch (OC 14:4): One may take another's Talis and bless on it.

Gra (DH Mutar): We learn from Pesachim 4b and Bechoros 18b.

Mishnah Berurah (13): The Pri Megadim says that whenever the owner is there, it is proper to ask permission. We do not rely on the Chazakah when one can easily verify it, and all the more so if one knows that he is particular.

Rema: The same applies to Tefilin. However, one may learn from another's Seforim (without his knowledge, lest he tear them while learning).

Taz (7): In CM 72:1, I explained that if one deposited Seforim with a Chacham, he may learn in them. The same applies if they were left with him for collateral.

Magen Avraham (10): The Rema and Nimukei Yosef connote that one may not learn at all from them. Regarding Bedikas Chametz (437:4), we say that one is happy to do a Mitzvah with his body or property, even if there will be a loss! There is different, for it is a small loss.

Gra (DH Aval): The Gemara forbids one who borrowed a Sefer Torah to lend it to another. When there can be a loss, we do not say that one is happy that people do a Mitzvah with his property (Bechoros 18b).

Mishnah Berurah (16): The custom is that one who finds a Sidur or Machzor in the Beis ha'Keneses takes it to pray in it. I do not understand this. Why is it different than a Sefer? (Pri Megadim)

Kaf ha'Chayim (31): Chesed la'Alafim says that many stumble in this. It is proper for every Yisrael to announce that he is happy for people to use his Seforim for a Mitzvah, and no one will stumble through him. Since the custom is to use Sidurim left around, one who leaves his Sidur in the Beis ha'Keneses shows that he consents that people use it, as long as it is returned to its place. One should not take it elsewhere, lest the owner seek it at the time and not find it, and be angry at the borrower.

Rema (CM 163:6): There was an early enactment not to take a Talis or Sefer out of a Beis ha'Keneses without the owner's permission.

Gra (105): Even though in a Beis ha'Keneses it is permitted (others may use it for a Mitzvah - PF), for people bring with this intent (Bava Metzi'a 29b), one may not (learn a new matter, lest this cause a loss. Also here, taking it outside is like a loss - PF.)

Shulchan Aruch (292:20): If Ploni deposited a Sefer Torah with Reuven, Reuven opens it once a year and reads in it. He may not open it for his own need, to read in it.

Rema: Also, he may not copy from it. This refers to an ignoramus. If Reuven is a Chacham who does not have such a Sefer, he may read or copy from it, for surely Ploni lent it with this intent. In a situation of Bitul Torah, when Seforim are not available, Beis Din can force one to lend his Seforim for others to learn in them, but they must pay for any depreciation.

SMA (45): We learn from one who deposited unwrapped coins. Therefore, if the Sefer was wrapped and sealed with an unusual seal, this shows that Ploni does not want Reuven to read in it. However, the Mordechai learns from a Medrash. This connotes that it is permitted even if we know that Ploni objects. However, the Rema omitted this, and said 'surely Ploni lent it with this intent', which connotes that if Ploni objects, it is forbidden. Also the Mordechai said only perhaps we learn from the Medrash.

Levush: A Chacham may read or copy from the Sefer if he does not have another such Sefer. Then we say that surely Ploni lent it with this intent. Further, when there is Bitul Torah and Seforim are not available, Beis Din can force one to lend his Seforim for others to learn from them, like it says in Medrash Mishlei, that we do not disgrace one who 'steals' Divrei Torah and copies them, as long as he pays for depreciation to the Seforim.

Rebuttal (SMA 45): First the Levush wrote that the Rema permits because we assess that Ploni consents. Then he wrote that Beis Din forces people to lend Seforim. Then he said 'like it says in the Medrash, as long as he pays for depreciation.' It seems that 'like it says...' refers to what he said above, that a Chacham may copy. His final words (paying for depreciation) are based on losses due to reading and copying from the Sefer. He contradicts himself, like I explained. This requires investigation.

Defense (Shach 35): The SMA understood his Rebbi (the Rema) well. It is difficult to say that the Rema argues with the Mordechai without a reason, especially since he brought the Mordechai without dissent in Darchei Moshe. Also a Tosefta (Bava Kama 7:3) says that even though he is called a thief (for stealing Divrei Torah), in the end he will become a leader of the Tzibur and bring merit to the Rabim and himself. According to the SMA, the Mordechai should have said 'or perhaps the reason is...' Also, according to the first reason, did Ploni expect Reuven to copy from it?! Also, the Mordechai should have said that Ploni expected him to read and copy from it. Why did he omit copying? Also, since he already gave a reason, why did he need to say 'perhaps the reason is...'? Rather, Ploni's intent permits a Chacham to learn from the Sefer, but not to copy from it. The Mordechai suggested that perhaps the Medrash is the source to permit copying. The Rema teaches that Beis Din can force one to lend his Seforim for learning and copying, even if the lender objects.

Gra (46): In Sa'if 7, we distinguish depositing coins with a moneychanger and a regular person. (Similarly, we distinguish lending to a Chacham and an ignoramus.) However, this does not apply if we know that the depositor objects. However, in a case of Bitul Torah, it is permitted in every case.

Gra (47): The Gemara (Kesuvos 50a) says that "happy is the one who guards justice and constantly does Tzedakah" refers to one who writes Seforim and lends them to others. Beis Din forces people to give Tzedakah (Kesuvos 49b).