Gemara

(Mishnah): Once, R. Gamliel and Chachamim were on a boat. He said 'the tithe (Ma'aser Rishon) that I will measure off later is given to R. Yehoshua, and I rent to him the area it rests on. Another tithe (Ma'aser Oni) that I will measure off later is given to R. Akiva, to acquire it on behalf of the poor. I rent to him the area it rests on.'

Bava Metzi'a 11a (Ula and Rabah bar bar Chana): One's field acquires for him only when he is standing by it.

Question (R. Aba - Mishnah): A case occurred in which R. Gamliel...

R. Yehoshua and R. Akiva were not by the field, yet they acquired!

Ula: You ask like one who has never learned.

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

R. Zeira accepted this answer; R. Aba did not.

Rebuttal (Rava) Question: Why didn't they acquire them through Chalipin? Surely they had a garment to do Chalipin!

Answer: Since R. Gamliel did not own the tithes themselves, only the Tovas Hana'ah (right to give it to the Levi or Oni of his choice), Chalipin does not work. Similarly, one cannot acquire something Agav (land) when the giver owns only Tovas Hana'ah. (Rather, R. Gamliel allowed them to acquire it, like one acquires Hefker. The field acquired it for them.)

Rejection: Chalipin cannot be used for gifts to a Kohen because the Torah said they should be given. Chalipin is a way of commerce. Agav is a method of giving, so it can be used.

Answer #2 (Rav Papa): The case of R. Gamliel is different. Since one (with intellect) gave the tithes, the field can acquire even if the owner is not there.

49b (Beraisa - R. Shimon): If the seller has the money and the buyer has the Peros, he (the seller) cannot retract because his money is in his (the buyer's) hand.

This means that the exchange for his money (the Peros) is in the buyer's hand.

Objection: Obviously, he cannot retract! (Meshichah was done!)

Answer (Rava): The case is, the seller was renting the buyer's attic (where the Peros were.) Chachamim enacted that Meshichah acquires (and not money), lest someone sell something in his attic, a fire will erupt, and he will not strive to save it. If the Peros are in the buyer's attic, the buyer will strive to save them!

Bava Basra 84b (Mishnah): If Reuven sold Peros to Shimon, and he (Shimon) measured but did not do Meshichah, he does not acquire;

If Shimon is clever, he will rent the premises (to acquire immediately).

Rishonim

Rif and Rosh (Bava Metzi'a 28b and 4:8): If the seller was renting the buyer's attic (where the Peros were), the buyer cannot retract. There is no decree that money not acquire, for the buyer will strive to save the Peros. This is according to R. Shimon. We infer that Chachamim agree.

Rosh: The buyer's attic does not acquire for him, since it is rented to the seller. (Therefore, he needs to acquire through money.) Similarly, if the buyer rented the seller's attic and lives there, there is no decree that money not acquire.

Rambam (Hilchos Sechirus 6:5): One's yard acquires for him, even if it is rented to someone else.

Rambam (Hilchos Mechirah 3:6): Chachamim decreed Meshichah for Metaltelim (lest the seller be slothful to save them). Therefore, if they were in the buyer's house, which was rented to the seller, Chachamim did not enact Meshichah, for they are in the buyer's Reshus. Once money was given, neither can retract.

Magid Mishneh: The Rambam is difficult. It seems that he holds that the buyer's premises acquire for him, even though they are rented to others, like he said in Hilchos Sechirus. This is not clear from his words.

Kesef Mishneh: No, the Rambam means that in this case there is no decree of Meshichah, and money acquires like Torah law.

Lechem Mishneh: If he acquires through his premises, why does the Rambam say that the money was paid? And why does he say 'therefore'? (It does not depend on what he said above!) It seems that the Rambam explains that we never abolish Kinyan through one's premises due to the concern lest the seller be slothful to save the item.

Rambam (7): Similarly, if the buyer rented the place of the sale item, neither can retract, even though there was no Hagbahah, Meshichah or Mesirah, for it became like the buyer's Reshus.

Rosh (Bava Basra 3:54): R. Tam says that his rights of acquisition do not depend on eating the Peros, rather, on ability to sell or give the property.

Question (Tosfos Bava Basra 51b DH b'Matanah): A renter acquires, even though he cannot sell the property!

Answer (R. Tam, in Tosfos): A renter has great rights. Even if he died during the rental period, his heirs keep the premises until the end of the period. If a woman sold property and died, her husband takes it back immediately.

Poskim

Shulchan Aruch (198:5): Chachamim decreed that money not acquire, lest one buy an item in the seller's Reshus, and the seller will be slothful to save the item from a fire. Therefore, if the item was in the buyer's house, which was rented to the seller, there is no decree, for the buyer can save it. Similarly, one who rents a place acquires the Metaltelim there if the place is guarded.

Question (SMA 9): Why does the Shulchan Aruch say 'similarly'? In the Reisha, he acquires through money. In the Seifa, the place must be guarded, for he acquires through the premises! This is not difficult for the Rambam. He wrote (Hilchos Mechirah 3:6) that Meshichah was not enacted when the item is in the buyer's house, even if it is rented to the seller. The Magid Mishneh explains that it is still like the buyer's Chatzer to acquire what is inside. The Shulchan Aruch understood the Rambam to say that he acquires through money because there is no concern for slothfulness.

Answer (Shach 5): The Shulchan Aruch teaches that similarly, one can acquire without Meshichah if he rented the seller's premises! The Rambam says that he acquires after giving the money, for this is the Kinyan. The Magid Mishneh (said 'this is not clear from his words', i.e. he) retracted.

Shach (6): Renting or borrowing premises suffices to acquire through them.