Midway through Bava Metzia 45b, the Gemara leaves behind the discussion of whether coins (or fruit) can be exchanged for gold coins to be brought to Yerushalayim to buy fruit and begins a discussion of kinyan chalipim.
As one of the members of my daf yomi group reminded me this evening, the issue of kinyan chalipim is first developed by the Gemara in Kiddushin which discusses whether it can be utilized to effectuate a marriage.
However, the concept of kinyan chalipim actually draws its origin from Megillas Rus (4:7). Today, the kinyan is most widely seen in traditional transactions such as buying/selling chametz. The mechanics are simple - the buyer and seller are essentially trading objects - the seller takes an object from the buyer and by so doing, the buyer acquires the item which the seller wishes to sell him. Rashi notes that this can even effectuate the sale of real property.
On today's daf, Rav and Levi have an interesting machlokes about whether a coin can be used as kinyan chalipim. This seems like a roundabout way of saying that an item is "bought", but the gemara explores the issue. Under this scenario the seller sells the item in exchange for an item (the coin) which is acquired as an item and not as a unit of money. R' Papa explains that the reason why one de'ah holds that money cannot be the subject of kinyan chalipim is because the one who is acquiring the money does not acquire a complete item (like a cup or handkerchief) since in the times of the gemara the money was only good for the realm of the current monarch and the coin which was acquired today might be valueless tomorrow.
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