(Mishnah): If one of the residents of a Chatzer forgot to give towards the Eruv, his house is forbidden [to transfer between it and the Chatzer] to him and to them (the Bnei Chatzer). Their houses are permitted to him and to them;

If they gave their Reshus to him, he is permitted [to transfer between his house and the Chatzer], and they are forbidden [even from his house];

If two [forgot], they forbid each other. This is because one person can give or receive Reshus. Two can give Reshus but cannot receive it.

(Gemara): Their [houses] are permitted to him and to them. [The case is,] he was Mevatel his Reshus [in the Chatzer. He is like a guest staying with them.]

Question: [If two in a Chatzer did not make an Eruv, can one be Mevatel to the other, and the latter does his needs, and] can then they switch back (the latter is Mevatel to the first, so he can do his needs)?

Answer #1 (Rav's Talmidim citing Rav): They can switch back.

Answer #2 (Shmuel): They cannot switch back.

Question: Our Mishnah opposes Shmuel! It teaches that their [houses] are permitted to him and to them. Is this not when he was Mevatel his Reshus to them?! [And the Seifa teaches that later,] if they gave their Reshus to him, he is permitted and they are forbidden!

Answer: He explains our Mishnah to discuss different cases [in the Reisha and Seifa].