For the last couple days the Gemara has been discussing the laws of khalipin-חֲלִיפִין, a transaction of exchange which is done symbolically with a kinyan sudar-קִנְיָן סוּדָר. If you ever gone to a traditional Jewish wedding, the groom accepts the obligations written in the ketutbah when he pulls upon a handkerchief or a napkin held by the Rabbi. There’s been disagreements what may one use for a kinyan sudar, a durable good, produce, or money. Who gives whom the sudar, the buyer or the seller? Finally on today’s daf TB Baba Metzia 47 the Gemara comes to the conclusion as well as giving us the book of Ruth as the biblical source for this law.

“…in exchange for that pleasure the owner of the item experiences from the fact that the one acquiring the cloak accepted it from him, he resolves to transfer ownership to him. This is unlike the acquisition of movable items by means of a transaction of land, where both are acquired simultaneously. Here, the transfer of ownership of the cloak effects the subsequent transfer of ownership of the land.

“The Gemara comments: This dispute between Rav and Levi is parallel to a dispute between tanna’im. The verse states: “Now this was the custom in former time in Israel concerning redemption and concerning substitution, to confirm all matters; a man drew off his shoe, and gave it to his neighbor” (Ruth 4:7). The verse is interpreted: “Redemption”; that is a sale. And likewise it says: “Neither shall be sold nor shall be redeemed” (Leviticus 27:28). “Substitution”; that is the transaction of exchange. And likewise it says: “He may neither exchange it nor substitute it” (Leviticus 27:10).

“With regard to the phrase “To confirm all matters; a man drew off his shoe, and gave it to his neighbor,” the baraita asks: Who gave the shoe to whom? Boaz gave his shoe to the redeemer, the closest relative of Elimelech, who had the right of first refusal to the land that Naomi, Elimelech’s widow, was planning to sell. The redeemer was transferring that right to the land to Boaz, who was acquiring it by means of his shoe. Rabbi Yehuda says: The redeemer gave his shoe to Boaz. The dispute between Rav and Levi is parallel to the dispute between the first tanna and Rabbi Yehuda.” (Sefaria.org translation) The buyer and not the seller gives the sudar.

Based upon the story in the book of Ruth, Rav Naḥman holds that the sudar must be a durable good just like the shoe in the story. “It was taught: One can acquire property through a symbolic exchange by using a vessel, even if it does not have the value of one peruta. Rav Naḥman says: The Sages taught that this symbolic exchange can be effected only by using a vessel, but not by using produce, i.e., any item other than a vessel. Rav Sheshet says: It can be effected even by using produce. The Gemara explains: What is the reason for the opinion of Rav Naḥman? The verse states: “His shoe” (Ruth 4:7), from which it is derived: With regard to a shoe and any other item similar to a shoe, i.e., a vessel, yes, the symbolic exchange can be effected; with regard to any item other than a vessel, no, it cannot be effected.” (Sefaria.org translation)

The Gemara concludes that the sudar must be a durable good, have the minimum valued of a prutah, and be clean. In other words, the rabbi is not allowed to use a handkerchief that he has blown his nose in.