Gemara

(Ula and Rabah bar bar Chanah): One's field acquires for him only when he is standing by it.

Question (R. Aba - Mishnah): Once, R. Gamliel and Chachamim were on a boat. He said 'the Ma'aser (Rishon) that I will separate is given to R. Yehoshua, and I rent to him the area it rests on. The Ma'aser (Oni) that I will separate is given to R. Akiva, to acquire for the poor. I rent to him its place.'

Answer #1 (a Chacham): The land did not acquire the tithes for them. Rather, they acquired them Agav (along with) acquiring the land!

Answer #2 (to Question 3:b - Rav Papa): The case of R. Gamliel is different. Since one (with Da'as, i.e. intelligence) gave the tithes, the field can acquire even if the owner is not there.

25b (Beraisa): One may keep things he found in a rockpile or an old wall, for he can say that it was from Kena'anim (from the days of Yehoshua).

Gitin 77b (Rava): The moment a Get is put in a woman's hand, she is divorced and simultaneously gets back her Yad (power of acquisition). Similarly, the moment a Get enters her yard, she is divorced and they are hers.

(Rav Ashi): A husband does not own his wife's hands! Really, Rava learned from the opinion (Kidushin 22b) that a slave can himself receive a Get of freedom from his master. A slave's hand is like his master's hand. Rava answered that when a Get is put in his hand, he simultaneously goes free and acquires his Yad. This also explains how a woman can receive a Get in her yard!

A man did not expect to survive until after Shabbos; he wrote a Get to his wife just before Shabbos. Rava said that he should give to her, for a gift, the place where the Get is resting. She should close the door there (on Shabbos) to acquire it through Chazakah. She acquires the place at the moment of divorce.

Bava Kama 12a (Beraisa): If one was buying slaves and Metaltelim, and he did a Chazakah to acquire the slaves, he acquires the Metaltelim.

(Rav Ika brei d'Rav Ami): The Tana holds that slaves are like Metaltelim. The Metaltelim acquired with the slave is the clothes he is wearing.

A moving Chatzer does not acquire Metaltelim inside it! We must say that the slave is tied up.

Kidushin 27a: A Beraisa says that a Mishnah (about acquiring a document with the field) illustrates Agav. This shows that the Metaltelim need not be Tziburin (in the land).

Question: Must the giver say 'acquire Agav (along with)'?

The Halachah is, we do not require Tziburin, but the giver must say 'Agav' and 'acquire'.

Rishonim

Rif (Kidushin 10b): The Halachah is, we do not require Tziburin, but the giver must say 'Agav', i.e. acquire with this land the Metaltelim in the place Ploni.

Rambam (Hilchos Mechirah 3:8): If one is Makneh land and Metaltelim, once the buyer acquired the land, he acquired the Metaltelim.

Rambam (9): This is if the Metaltelim are in the land. If they were elsewhere, he must say 'acquire Al Gav the land.'

Rebuttal (Ra'avad): The Gemara requires saying Agav even if the Metaltelim are Tziburin. It concluded that we need Agav, but we do not need Tziburin. According to the Rambam, sometimes we need Tziburin, but not Agav!

Magid Mishneh: Perhaps the Rambam explains that the Gemara asked whether one must say Agav when the Metaltelim are not Tziburin. The Rif suggests like this, for he explains the need to say Agav only in this case. However, it is not certain that he holds like the Rambam.

Ran (Kidushin 10b DH v'Hilchesa): If the Rambam explains that the question was whether one must say Agav when the Metaltelim are not Tziburin, he should have said that the Chatzer is guarded or he is standing in it! This is clear from Bava Metzia 11a, and the Rambam himself rules like this (Hilchos Zechiyah 4:9).

Kesef Mishneh: The Gemara already concluded that we do not require Tziburin. It asked whether the giver must say 'Agav', and answered 'we do not require Tziburin, but the giver must say Agav'. It repeated that we do not require Tziburin to teach that he needs to say Agav only in this case. Perhaps the Rambam relied on what he wrote in Hilchos Zechiyah. This is how the Ran explained the Rambam. I say that if the Kinyan is due to Chatzer, the Rambam would not have taught it here! Rather, when they are not Tziburin, Agav is needed to clarify that he is Makneh them with the land. When they are Tzibur, this is clear without saying Agav.

Rambam (Hilchos Gezeilah 16:8): A Chatzer acquires for its owner even without his knowledge. If a treasure was found in an old wall, why doesn't the owner of the wall acquire through is Chatzer? This is because he or others do not know about it. It is lost from him and others.

Rosh (2:9): A Chatzer does not acquire something (e.g. a Kena'ani's object) that might never be found. Further, it does not belong to the Yisrael who inherited this wall (in the days of Yehoshua), for the spoils were divided among all of Yisrael. Why didn't the owner's Chatzer acquire for him after Yisrael despaired? His Chatzer is no better than his hand. If something came to his hand before despair, he does not acquire it afterwards, for it came to his hand b'Isur (in a way that it was forbidden for him to keep it). Also his Chatzer does not acquire in such a case.

Hagahos Ashri: Avi ha'Ezri says that in such a case, we cannot say that one's Chatzer acquires for him without his knowledge and the first one who bought the wall acquired. This is unlike normal Metzi'os. It never crossed the mind of the buyer to acquire what is inside. Therefore, he acquires only what he wanted to acquire. His Chatzer acquires for him only what will enter it afterwards. A case occurred in which Levi bought what was assumed to be tin from a Nochri smith, in order to cover his roof. He sold it to David. It was found to be silver covered with tin. The Re'em exempted David, for Levi did not acquire the silver, since he did not know about it. R. Tam agreed.

Poskim

Shulchan Aruch (CM 198:5): One who rents a place acquires the Metaltelim there if the place is guarded.

Shach (7): Renting suffices to acquire only if afterwards the owner sold or gave the Metaltelim. If both are at the same time, it does not work.

Question (Ketzos ha'Choshen 2): A woman receives her Get and Chatzer at the same moment! Perhaps this is because her Chatzer works like a Yad, but a man's Chatzer is like his Shali'ach. However, they work together even through Shelichus, e.g. R. Gamliel and Chachamim. It was not through Yad, for they were not near the premises. The Mordechai explains that it was Da'as Acheres Makneh (someone gave it) because he gave to them the Chatzer at the same time. The Shitah Mekubetzes (Bava Metzia 25b DH Lo Nitzrechah) says that one who acquires a Chatzer acquires what enters it afterwards, but not what was there from before. This is like the Shach.

Rema (202:2): If one is Makneh land and Metaltelim, some say that even if the Metaltelim are in the land, he must say 'acquire Agav the land.' This is primary.

SMA (4): This is when the land is not guarded. If it is guarded, even without Agav he acquires through Kinyan Chatzer.

Rebuttal (Shach 3): The Ran, Magid Mishneh and Ra'avad say that even a guarded Chatzer acquires only what enters it after he owns the Chatzer, but not what is acquired at the same time as it. It seems that the Tur and Rema agree.

Pnei Yehoshua (Kidushin 27a DH Mekomo): In Kidushin, the Gemara assumes that R. Gamliel gave to the Chachamim Agav the land. In Bava Metzia, Amora'im argue about this! Rav Papa and Rav Ashi conclude that they acquired through Chatzer! Some Poskim rule like them, for they are Basra! It seems that Rav Papa and Rav Ashi came to show that one may not settle either of two questions from the episode. Even if we cannot learn that Agav does not require Tziburin (because they acquired through Chatzer, not Agav), we cannot learn that a Chatzer acquires even if the owner is not there, for perhaps this is only with Da'as Acheres Makneh. Perhaps they agree that the primary Perush is that he gave Agav, for if not, he should have rented the land to them before giving the tithes. This is not only according to the Shach, who says that Chatzer does not acquire Metaltelim in it given at the same time. Even unlike the Shach, the Chatzer does not acquire what was already there, so after renting the land, R. Gamliel needed to again say that he gives the tithes to them. It is much better to say that the Beraisa discusses Agav, in which one must mention the Metaltelim before the land. The Shach is difficult, for Gitin 77b clearly shows that Chatzer acquires what is given at the same time!) Even if one distinguishes, Bava Kama 12a and Bava Metzia 11b show that Chatzer acquires (what is given at the same time).