Siman 411

One who said to his agent while it was still day: "Establish an eruv for me to the west of my house," and he went and established an eruv for him, and he himself went for his affairs afterward to the east of his house, and was there during bein hashemashot at the time of the acquisition of the residence. If the agent placed the eruv further from his house to the west so far that from the eruv to the place where he stands to the east of his house it is more than two thousand—it is understood automatically that the eruv is void, and he remains with the techum of his house. But if it is in a manner that from the eruv to the place where he stands it is not more than two thousand, even though he also stands within two thousand of his house—nevertheless, he has the techum of his eruv. And we do not say that since he now stands to the east of his house it is not pleasing to him at all with his eruv which is to the west of his house—we do not say so.

And we have already written in siman 409, section 7, that regarding the techum of his house, we do not say that he may not move from his place, as we do in the case of one who recognizes a tree or a fence, because he never removes his mind from his house, see there; and Rashi, of blessed memory, wrote this reasoning, see there. And some say the reason that he did not lose the techum of his house is not because of this, but rather because it is the agent who erred, for he should not have placed the eruv so far away, and automatically the agency is voided. (The R"ri) However, in siman 408 it implies explicitly like Rashi, see there. (See the Taz that there is a practical difference between the reasons when he himself did so, and placed an eruv in the west and went out to the east, see there)