Bava Metzia 47 - How is the "Exchange" Acquisition Performed?
In the passage from the Book of Ruth which serves as the source for "exchange" acquisition it is not clear who gave the shoe to whom.
Rav says that the acquisition is performed with the utensil (kerchief) of the buyer. Though the buyer gives his utensil to the seller in addition to the money that was agreed upon, it is in the interest of the buyer to do so, so that the transaction be finalized immediately.
Levi says that it is performed with the utensil of the seller. In exchange for the pleasure that the seller by the buyer accepting the kerchief from him, he resolves to grant the ownership.