Kiddushin, Chapter 1, Mishnah 6. This is the final mishnah in the chapter dealing with the laws of acquisition, and it discusses a method of acquisition known as chalifin, exchange. Chalifin is the situation in which one item is traded for another - a form of barter. The mishnah teaches that barter works with all types of movable property, with the exception of money, that is, coins that actually serve as a medium of exchange for purchasing items.
The text of the mishnah:
"Kol hanishom damim b'acher, keivan shezachah zeh nis'chayev zeh b'chalifo" - the Gemara explains that "kol hanishom damim b'acher" refers to anything whose value must be assessed and appraised, as opposed to a coin, whose value is fixed in advance and needs no appraisal. In other words: any item whose worth has to be evaluated in a sum of money. "Keivan shezachah zeh" - from the moment one person takes ownership of the one item, "nis'chayev zeh b'chalifo" - the other party, who traded something in return for that item, becomes the owner of the item opposite it by virtue of the exchange.
How does chalifin work?
The mishnah gives an example: "Hechlif shor b'farah, o chamor b'shor" - from the moment one person takes ownership of one of the animals, the other becomes the owner of the item opposite it by virtue of that same exchange. In other words: if I take ownership of your donkey, with the understanding that I will receive the donkey and you will receive my ox, then my very act of taking hold of the donkey automatically transfers the ox to you - even if you never touch the ox and are nowhere near it. This is how the acquisition of chalifin works, with everything except a coin: there is no way to acquire coins and take ownership of them through the method of chalifin.
Reshus hagavoha and reshus hahedyot:
"Reshus hagavoha b'chesef" - ownership of things belonging to the Most High, items consecrated to the Temple treasury, is acquired through the transfer of money alone.
"U'reshus hahedyot b'chazakah" - when dealing with something that is not consecrated property, one must take physical possession of the item, whether by pulling it or by another method of acquisition.
Ordinarily we say that someone who wishes to acquire a movable item and take ownership of it must acquire it through meshichah, physically taking hold of it, while handing over money alone is not sufficient. With consecrated property, however, the money alone is enough.
Amiraso l'gavoha k'mesiraso l'hedyot:
Another concept in the laws of consecrated property: from the moment a person verbally declares that he is giving something to the Most High, to the Temple treasury, that statement is equivalent to an actual physical transfer between one ordinary person and another. When an ordinary person hands an item to his fellow, meshichah or some other physical form of acquisition is required; whereas the statement a person makes that he is giving something to the Temple treasury is equivalent to a physical handing over from one person to another. The statement alone is like the physical transfer, when consecrated property is involved.
In summary: in this mishnah we learned the laws of acquisition through chalifin - which works with anything that is assessed and appraised in monetary terms, except for a coin, and once one party acquires the item, the other becomes the owner of its counterpart even without having touched it. We also learned the difference between reshus hagavoha, which is acquired with money, and reshus hahedyot, which is acquired through chazakah, as well as the principle that a statement to the Most High is equivalent to an actual transfer to an ordinary person.