Siman 201

The acquisitions that the merchants are accustomed to acquire with, and it is a fixed custom in the city, one acquires with these acquisitions a complete acquisition if they performed the acquisition after the price of the sale was settled, and neither of them can retract, for custom is a great principle in monetary laws. Even in the time of the sages of blessed memory it was so, and this is what they said [Bava Metzia 74a]: Situmta acquires regarding "He who punished," and in a place where they are accustomed to acquire with this a complete acquisition, it acquires a complete acquisition. Its explanation is that there was a custom in their days that the buyer marked a mark on the barrel of wine that he bought so that he would have a known sign that it is his. Even though he did not give money at all, nevertheless, anyone who retracts after he marked receives "He who punished." And if the custom of the land is to acquire with the mark a complete acquisition, the sale is acquired and neither of them can retract, and the buyer is obligated to give the money. This is specifically when he marked in the presence of the seller, or if he said to him, "Mark your purchase" or "Go and acquire," and presumably his intention was that he should acquire with this according to the custom. See what I wrote in siman 204, section 4.

And so too, anything that the merchants are accustomed to use for acquisition, such as in a place where they are accustomed that the buyer gives a deposit which they call "hand-geld," or in a place where they are accustomed that they hand over the key to the buyer to acquire the merchandise placed there, and so too anything similar to this. And there is no distinction in this between land and movable property if it is a fixed custom [Shach]. And there are some of the later authorities who hold that an acquisition based on custom is only a rabbinic acquisition, but the Ramban wrote that it is an acquisition from the Torah [Chatam Sofer], and it is effective even regarding a firstborn and the betrothal of a woman. Aside from this, the primary ruling is that a rabbinic acquisition is effective even for a matter that is from the Torah, for so it is explained in the Yerushalmi, as I wrote in siman 198, section 7.

Some say that a kinyan practiced by merchants is only effective for a kinyan that is effective according to the law; however, for that which is not effective according to the law, such as a thing that has not yet come into the world, or something that has no substance, or something that is not in his possession, no custom is effective. And there are those who disagree on this, for if it is an established custom, it is effective for everything they practiced. And behold, we see that they practiced to transfer all types of leases and arendas which are not in his possession and contain things that have not come into the world at all, and nevertheless, the simple custom is that the kinyan or the deed is effective for them [ibid.]. And the early authorities also disagreed on this, for the Rosh in a responsum and the Maharam of Rothenburg at the end of the Hagahot Mordechai of Shabbat wrote that it is effective, and the Ri wavers on the matter [and what the Chatam Sofer wrote regarding his intent does not seem so to one who examines it there, and look closely]. And from the Rashba, whom the Maggid Mishneh cited in chapter 7 of Sale, it also implies that it is effective in every case since such is the custom, and it is a complete law. And such is the primary ruling, and even regarding asmachtas there is a kinyan if they practiced so, and all the more so if the custom is by virtue of the law of the kingdom, for it is obvious that it is effective in every case, for the law of the kingdom is the law [the Gara]. And all this is after the finalization of the price, but a kinyan prior to the finalization is not effective at all, and one must perform the customary kinyan once more after the finalization [see in the Mordechai, chapter 3 of Gittin regarding priestly acquaintances, and in the Maharik, root 76, and in the Yam Shel Shlomo, chapter 8 of Bava Kamma, siman 60].