In the Mishna on our daf, Rabbi Eliezer teaches that you can give money to the grocer or baker who live with you in the same courtyard and ask them to include you in the communal eiruv. The hakhamim argue, since the general principle in halakha is that one cannot transfer property by simply paying money; a symbolic act of taking ownership, such as lifting the object up is essential.
The Gemara asks why Rabbi Eliezer does not seem concerned that the money will not accomplish its expressed purpose, which is purchasing a share in the eiruv. After a number of suggestions are raised, the Gemara concludes that Rabbi Eliezer puts this case into the same category with a number of other cases where purchase by means of money is permitted by the Sages.
In truth, on a Biblical level, money is an acceptable means of purchase; it is only a Rabbinic decree that a symbolic act is essential. The Sages were concerned lest a person would purchase an amount of grain from the seller, and when he came to collect it the seller would claim that a fire had destroyed the particular grain that had been sold to him. By forcing the parties to include a symbolic act of purchase in the sale, it would be clear to all involved what had been sold. The effect of this ruling is that under normal circumstances, even if money has changed hands, still either the buyer or the seller can back out of the agreement (although it is considered improper to do so). On the other hand, if the purchaser has performed a symbolic act of purchase – even if no money has changed hands – the object now belongs to him fully. Of course, he now has to pay the seller the money that he owes him.
Rabbi Eliezer believed that in this case – as is true in four other specific cases in the Gemara – an exception would be made in order to make it easier to create an eiruv.
The halakha, however, follows the hakhamim, and money cannot be used to participate in an eiruv, if it is given to the local grocer or baker.
And the Rabbis concede with regard to all other people, apart from grocers and bakers, that if one gave them money for the food of an eiruv, his money confers possession upon him, as one may establish an eiruv for a person only with his knowledge and at his bidding. With regard to a grocer or baker, the person giving the money does not intend to appoint the grocer or the baker as his agent and the money itself does not effect an acquisition, and consequently, he did not accomplish anything. With regard to anyone else, however, there is no doubt that he must have intended to appoint him his agent, and his act is effective.