Gemara

25b (Mishnah): One may keep things found in an old wall.

(Beraisa): The finder can say that it was from Kena'anim.

The case is, they are very rusted. (Therefore, we are not concerned lest a Yisrael owned the wall and left it inside.)

26a (Mishnah): If Reuven rented his house to others, one may keep what he finds even in the house.

Question: We should assume that it belongs to the last tenants!

Answer (Rav Menasiya bar Yakov): The house was rented to three Nochrim.

102a (Mishnah): The dung belongs to Reuven (who owns the house). Shimon (the renter) gets only (ashes) from the oven.

Question: If Shimon rents the Chatzer and the dung is from his own animals, why does Reuven get it? If Reuven did not rent out the Chatzer and the dung is from his animals, obviously he gets it!

Answer: The case is, it is Reuven's Chatzer, and dung from others' animals.

(Beraisa): Reuven gets dung deposited in the pen and the Chatzer.

(Abaye): This means that dung in the pen in the Chatzer belongs to Reuven (even though Shimon rented the Chatzer).

(Rav Ashi): This teaches that rental of a Chatzer does not rent the pen inside.

Rishonim

Rif and Rosh (Bava Metzia 59a and 8:28): The house owner gets the dung, i.e. when he did not rent out the Chatzer. The dung is from others' animals.

Rambam (Hilchos Sechirus 6:5): Dung in a Chatzer from the renter's animals belongs to the renter. Dung of others' animals belongs to the owner. One's yard acquires for him, even if it is rented out.

Rebuttal (Ra'avad): We established the Mishnah to be when he did not rent out the Chatzer, and the dung is from others' animals. Had he rented it out, the renter would get it, just like a buyer acquires by renting the premises.

Defense #1 (Magid Mishneh): The Rambam explains the conclusion to discuss Reuven's Chatzer and others' animals, even when it is rented out. The Gemara said 'it is Reuven's Chatzer.' It did not say 'he did not rent out the Chatzer', like it said above. The Ra'avad explains that the renter has permission only to pass through to his house.

Rebuttal (Gra CM 260:13): We cannot infer from the 'omission' of 'he did not rent out the Chatzer.' Nor did it say 'he rented it out'! Rav Ashi deduced that he does not rent the pen in the Chatzer (for if so, he would acquire the dung)!

Answer #1 (Ketzos ha'Choshen 1): Normally, an owner grants rights to acquire to the renter. However, if one rented a Chatzer and kept the pen, surely he wanted to acquire the dung for himself.

Answer #2 (Maharit 2 CM 83): The Rambam explains that Rav Ashi learned from 'and in the pen in the Chatzer'. In the Chatzer is extra, to teach that a renter acquires a pen in a Chatzer only if it was explicitly included.

Defense #2 (Shach CM 313:1): We cannot learn about acquiring Hefker from a sale, in which another party transfers ownership. Alternatively, a renter acquires only what he (knows about and) wants to be there.

Rosh (Bava Metzia 2:9): The one who rented his house to Nochrim did not acquire (through his house) the Aveidah they left behind after they left. This is because he always rented it out, so he was not destined to find the Aveidah.

Hagahos Ashri: The first one who bought the wall (in which a Kena'ani left something) did not acquire it. Avi ha'Ezri says that this is because one's Chatzer acquires for him in the future without his knowledge. It acquires for him what is already inside only if he intends for this.

Question (Tosfos Bava Metzia 11b DH v'Chi): How does the owner of the Chatzer acquire the dung? He does not guard it, for the renter is in the Chatzer!

Answer (Tosfos): The case is, also the owner has a door open to the Chatzer.

Poskim

Shulchan Aruch (260:4): If Reuven lived with Shimon in Shimon's house and paid rent, if a lame deer entered, both acquire (Rema - like a Chatzer of partners).

Shach (1): Tosfos (Bava Metzia, ibid.) holds that only Shimon acquires.

Shulchan Aruch (313:3): Dung in a Chatzer from the renter's animals belongs to the renter. Dung of others' animals belongs to the owner. One's yard acquires for him, even if it is rented out.

Question (SMA 5): This is like the Rambam. Above (260:3), the Shulchan Aruch rules like the Rosh (Teshuvah 1:1), Tur and Rashi, who say that the renter acquires!

Answer #1 (Shach 1): The Toras Chaim answered (that here is different, for the Chatzer was not rented for the sake of animals), but his words are difficult (some delete this from the Shach). The Rambam's opinion is primary. The SMaG, Magid Mishneh, Nimukei Yosef and Tosfos (ibid.) support him.

Rebuttal (Beis Meir, in Teshuvas R. Akiva Eiger 133 DH v'Hinei Kavod): Only the Magid Mishneh supports the Rambam! Tosfos holds that the Chatzer is not rented. He asked why the owner acquires, because the renter walks through. The Rif and Nimukei Yosef explain like Rashi, that the renter acquires. We follow them, the Ra'avad, Rosh and Tur against the Rambam.

Answer #2 (Ketzos ha'Choshen 1): Normally, an owner cedes rights to acquire. Here, he kept the pen. Surely wanted the dung for himself.

R. Akiva Eiger (ibid., DH v'Hinei l'Mah): Tosfos holds that a room rented out does not acquire for the owner. The renter can keep the owner out, so it is not guarded for the owner. Even though the Rambam did not specify that the owner has a door open to the Chatzer, perhaps he requires this. He often cites the Gemara verbatim. However, in Hilchos Mechirah (6:6), the Rambam says that Chachamim did not enact Meshichah when the seller was renting the buyer's house. The Magid Mishneh says that the buyer acquires through his house. The Rambam changed from the Gemara (which discussed a rented attic) to teach that this applies even when everything is rented. However, the Shach (198:5) says that the Kinyan is through money. I.e., because the buyer does not live there, his Chatzer does not acquire!

Shach (1): Tosfos requires that the owner has a door open to the Chatzer. It seems that the Rambam other Poskim do not require this. It seems that it suffices that it is guarded, and it is his Chatzer.

Rebuttal (R. Akiva Eiger, ibid. DH b'Ofen): The Rambam holds like the second answer in Tosfos (Gitin 79b DH Pnimis), that the renter is like the owner's Shali'ach (to guard it for him). According to the Shach, the Rambam should hold that a buyer's house acquires when it is rented to the seller!