There are still some things that I am not clear about in Eruvin page 82. However I thought to at least jot down the basic issues.
The Mishna there says one can make a eruv for boundaries the day before Shabat and the person has to accept it while still day. The Gemara asks that that sounds like there is "no choice that works in reverse", for if there would be choice, then one could make the eruv in the day and at night the person that it is made for can accept it and it would be known in reverse that it was ok with him even during the day. Rav Ashi answers and says really there is choice and here we are talking about when they let the person know that the eruv is being made but he did not accept it until night.
The problem is that אין חבין לאדם שלא מדעתו. One can not make an acquisition for another that causes him harm unless it is with his knowledge. [And an eruv is in that category since he loses the right to walk on Shabat in all directions except 2000 amot from the eruv.]
Now since from the Torah there is "no choice in reverse", the answer of Rav Ashi is only going according to the opinion "there is choice". The issue is not if there is choice when it comes to a eruv. Rather the question is can it be revealed after the fact that the direction chosen to place the eruv was good for the person--i.e. the direction he wanted to walk it on Shabat. It is not a question if the eruv is valid in one direction or another Rather if this direction is a good thing for that person.
Thus the Rif and Rosh both leave out the answer of Rav Ashi. That is how Rav Shach explains the Raavad over in laws of eruv. [chapter 6:23]
The question in Eruvin 82 is about if that direction is good. That is different than the issue of a person that makes his own eruv and says, "If the wise man comes in that direction, then my eruv is in that direction. And if in the other direction, then my eruv is in that other direction." Now in this last case, there is no doubt that one can make his own eruv even in a direction he does not want to go. So the only question is "retrograde choice " in eruv which we know is ok. That is different from one making an eruv for someone else, and that someone else deciding later on shabat that that is the direction he wants to go.
The Raavad is brought in the Rashba and Ritva on page 82b. The Rambam seems to be going with Rashi. The issue is this: retrograde choice is valid for laws from the sages but not for laws from the Torah. So the way Rashi and the Rambam look at this is that the one for whom the eruv was made can decide later which direction he wants to walk in and that in reverse means that the eruv that was made from him was good. To the Rif, Rosh and Raavad this would only work for the the law of eruv. Not for something which is a liability for which the law of retrograde choice is that it would not work.
