Gemara

(Mishnah): One receives Metzi'os that his small children find.

(Shmuel): Chachamim said this because a child immediately brings what he finds to his father. (He picks it up with this intent.)

Inference: Shmuel holds that a minor cannot acquire mid'Oraisa!

Question (Beraisa): If Reuven (a rich man) was hired to harvest, his son may walk after him and take Leket (what falls during harvesting; it is left for the poor). If Reuven receives a fixed percentage of the harvest, this is forbidden;

R. Yosi permits in either case.

(Shmuel): The Halachah follows R. Yosi.

Summation of question: If a minor cannot acquire, he takes for his father. This should be forbidden, for his father is rich!

Answer (Rava): Chachamim enacted that it is as if the son acquires. This helps Oniyim; when an Oni is hired, his son may collect after them.

Shmuel argues with R. Yochanan

(R. Yochanan): 'Small' and 'big' in the Mishnah do not refer to minor and adult. Rather, any child fed by his father is called small. Any child who feeds himself is called big.

Gitin 30a (Mishnah): If Reuven lent money to David (a Kohen, Levi or Oni), and stipulated to keep the Terumah, Ma'aser Rishon or Ma'aser Oni that he would have given to David, and deduct its value from the loan, he may do so, without concern lest David died or (if he was poor) became rich.

Question: David never received the tithes! (Reuven must give them!)

Answer #1 (Rav): The Mishnah discusses Makirei Kehunah or Leviyah (Reuven always gives his tithes to David). Once Reuven separates it, it automatically belongs to David!

Answer #2 (Shmuel): The case is, Reuven asks a third party to be Zocheh (acquire) the tithes on behalf of David.

Answer #3 (Ula): The Mishnah is R. Yosi, who says that Chachamim enacted that it is as if one acquired, even though (mid'Oraisa) he did not acquire.

Shmuel and Ula did not answer like Rav, for the Mishnah does not mention Makirei Kehunah or Leviyah. Rav and Ula did not answer like Shmuel, for the Mishnah does not say that another acquires for David. Rav and Shmuel did not answer like Ula, who establishes the Mishnah like an individual's opinion.

Bechoros 17a (Mishnah): If twins were born at the same moment to Reuven's sheep (its first birth), surely one was first. He keeps one, and gives the other to a Kohen. He gives Matanos (foreleg, jaw and stomach) to a Kohen;

R. Yosi exempts from Matanos.

18a (Rava): R. Yosi holds that (if Reuven kept the Bechor,) Chachamim consider it as if the Kohen acquired the Bechor, and after it got a Mum, traded it with Reuven for the other animal.

(R. Elazar): Even R. Yosi obligates Matanos when the Kohen gets nothing (e.g. one of the twins was a female).

Question: This is obvious! In the Seifa (18b), R. Yosi exempts 'whenever the Kohen received Chalipin (something in place of the Bechor)'!

Answer: One might have thought that R. Yosi addresses R. Meir (the first Tana) according to R. Meir's reasoning. I decree to exempt even when the Kohen received nothing, lest people come to shear or work with a Safek Bechor. Even if you do not decree, you should exempt when the Kohen received Chalipin!

Rishonim

The Rif and Rosh (Bava Metzia 5b and 1:33) explain the Mishnah in Bava Metzia like R. Yochanan.

Rambam (Hilchos Ma'aser 7:6): If Reuven lent money to David (a Kohen, Levi or Oni) and stipulated... when Reuven separates Terumah, Ma'aser Rishon or Ma'aser Oni, he gets another to acquire for David. If Reuven normally gave his Terumah or Ma'aser to David, no one else need acquire for David.

Magid Mishneh: Rav, Shmuel and Ula all agree about the Halachah. They argue only about how to explain the Mishnah.

Chasam Sofer (Gitin 30a DH k'Rebbi): Regarding a loan, the Rambam rules unlike R. Yosi, but regarding a Bechor mixed with Chulin, he rules like R. Yosi and exempts from Matanos. He also rules like R. Yosi regarding an Oni's son collecting Leket, for the Aniyim desire this. Regarding the loan, and an Oni knocking olives off a tree (Gitin 59b), there is no pressing need for an enactment as if he acquired. Even so, R. Yosi enacts; the Halachah does not follow him. Shmuel holds like R. Yosi in every case, even when the Kohen received nothing. We must say that not only can Chachamim make Hefker; they can even put something in one's Reshus. They learn from the verse that equates leaders to Avos (Gitin 36b). This is like R. Yochanan ben Berokah (Bava Basra 130a), who says that one can bequeath to whom he wants. Shmuel did not establish our Mishnah like R. Yosi, for if so a Stam Mishnah in Gitin would be followed by a Machlokes in Bava Basra, and the Halachah is unlike the Stam. Some Poskim hold that this means the Halachah surely (not just perhaps) is unlike the Stam.

Note: For two Masechtos, Ein Seder l'Mishnah (we do not know which was taught first - Bava Kama 102a). I did not find anyone who disagrees. Why didn't he bring a proof from Bava Basra 131a? Noson ha'Bavli proved from a Mishnah in Kesuvos that our Stam Mishnah is like R. Yochanan ben Berokah; Rebbi did not admit. Why didn't either prove this from our Mishnah in Gitin?

Rosh (Bechoros 2:4): The Halachah follows R. Meir.

Mordechai (Bava Basra 657): In the Yerushalmi, Amora'im argue about whether or not the Gabai (overseer of Tzedakah) must borrow for the sake of Aniyim when the Tzedakah box is empty. Or Zaru'a rules that we tell the Gabai to borrow. Avi ha'Ezri says that since it is called a loan, when there is money in the pouch later, it is repaid. He need not ask permission from those who give. My Rebbi says so even about one who lends to collect from Tzedakah and Ma'aser that he normally separates. This is unlike the Mishnah which requires asking permission from the Oni's heirs. If he lent in Beis Din; he does not lose his loan. All the more so when one lends to the Tzedakah box, which is for all Aniyim! I disagree with those who learn from Gitin that he needs permission from the donors. The Yerushalmi is not concerned lest all the Aniyim became rich. Even concerning one Oni, we are not concerned unless we find out!

Poskim

Shulchan Aruch (YD 257:5): When the Tzedakah pouch is lacking and the overseer needs to lend to it (from his money), when there is money in the pouch later he can be repaid. He need not ask permission from those who give.

Rema: If Reuven separates Ma'aser and lent to an Oni (David) from Reuven's own money, he may consider the loan to be Ma'aser and keep that amount of Ma'aser for himself. This is only if the Oni is still alive. If he died or became rich, he need not pay, for he was an Oni at the time; Reuven may not keep Ma'aser that was still owes, for we do not separate based on what was lost. One need not be concerned lest the Oni became rich, unless he finds out.

Shach (12): Reuven lent on condition to be repaid from gifts that he will give to David in the future. Therefore, he can separate Ma'aser or Tzedakah and consider it as if he gave to him.

Shach (13): Sefer ha'Terumos, the Bartenura and the Rambam require another to acquire on behalf of David, and then Reuven is paid from it, in order to fulfill "Nason Titen" (the Mitzvah to give Tzedakah). If he normally gives to David, this is like Makirei Kehunah and Leviyah, and Zechiyah is not needed. This is difficult. Ula established the Mishnah like R. Yosi, who says that Chachamim enacted that it is as if he acquired. If so, no Zechiyah is required. Tosfos says that even though Rav and Shmuel did not want to explain the Mishnah like an individual, the Halachah follows R. Yosi, like Shmuel himself says in Bava Metzia. Perhaps this is why the Rema did not mention that one must be Zocheh. Sefer ha'Terumos, the Bartenura and the Rambam must explain like the latter answer in the Rashba, that Rav and Shmuel hold that R. Yosi does not enact in a case like this.

Noda b'Yehudah (YD 1:73): If one gave Ma'aser to Aniyim above a 10th of what he already earned, perhaps he may not deduct the excess from his obligation to tithe future earnings. The Rema's Heter is when one lent to the Oni. If he gave for Tzedakah, there is nothing from which to deduct later! Even when he lends, it is not a normal loan, for the Oni need not pay even if he will get rich and the Ashir cannot separate due to him. One who normally lends like this need not tell the Oni; Stam, it is with this intent. Even if he is not explicitly Mezakeh, we rely on the Rema, who says that Chachamim enacted that it is as if he acquires. I say that the Rambam agrees that the Halachah follows R. Yosi, which is a leniency for the Nitva, i.e. the lender. Since he is Muchzak, we rely on R. Yosi. Also, Ma'aser of money is mid'Rabanan. The Mishnah discusses Ma'aser mid'Oraisa, for which one must fulfill the Mitzvah to give, so we do not rely on R. Yosi.