Rashba BM 74: Minhag uproots halacha

In today’s daf (Bava Metzia 74a), we read about the situmta:

אמר רב פפי משמיה דרבא האי סיטומתא קניא למאי הלכתא רב חביבא אמר למקניא ממש רבנן אמרי לקבולי עליה מי שפרע והלכתא לקבולי עליה מי שפרע ובאתרא דנהיגו למקני ממש קנו:

In English:

R. Papi said in Raba's name: The mark [on the wine-barrels]3  gives possession. In respect of what [does it effect a title]? — R. Habiba said: In respect of actual possession.4  The Rabbis said: For the acceptance of the curse.5  And the law is that [it gives possession only] in respect of submission to the curse. But where it is the usage that this gives actual possession, it does so [with full legal recognition].6

Rashi renders situmta as in the translation above, a mark on wine barrels.

סיטומתא - חותם שרושמין החנונים על החביות של יין שלוקחין הרבה ביחד ומניחין אותו באוצר הבעלים ומוליכין אותן אחת אחת למכור לחנות ורושמין אותם לדעת שכל הרשומות נמכרות:

The Rosh cites Rashi and also adds the explanation from Rabbenu Chananel:

“Rashi explains (d”h situmta) that it is a mark the wine-sellers make on the barrels they purchase from the householders. And Rabbenu Chananel explains (see Tosafot haRosh d”h Situmta) that this is the manner in which merchants conduct themselves at the end of a sale, they shake one another’s hand, and with this the sale is finalized. And anything like this [as well works], meaning in whatever manner they are accustomed to finalize the purchase. Such as in a place where the purchaser gives a single coin to the seller, and in this way they finalize the purchase.”

Nimukei Yosef only gives Rasi’s explanation, but also cites an interesting Rashba:

“The Rashba za’l wrote that we deduce from here that minhag overrides the halacha in anything like this. For in any monetary matters, based on the minhag we acquire and have things acquired. Therefore, via any device by which the merchants are accustomed to purchase, we [validly] acquire.”