(Mishnah - R. Yehudah): If Reuven was walking towards another city for which one needs an Eruv to go there, and Shimon met him and persuaded him to go back, Reuven may go there. The rest of his city is forbidden.

R. Meir says, anyone who could have been Me'arev, but was not, is in Chamar Gamal.

(Gemara): There are two explanations [of R. Yehudah. One is,] if [Shimon] said [to Reuven] "I will be Me'arev for you and the people of your city'', and he was Me'arev for the people of his city, but he was not Me'arev for Reuven, Reuven is forbidden in that direction, and permitted in all other directions in his city (like one who was not Me'arev), and they are permitted in that direction, and forbidden in all other directions in his city (like one who was Me'arev).

There is another explanation. [Shimon] said [to Reuven] "I will be Me'arev for you and the people of your city'', and he was not Me'arev, not for him and not for the people of his city. Reuven may go in that direction, for he was Me'arev through his feet before Shabbos, and they are forbidden in that direction, and permitted in all other directions in his city.