How can someone purchase or take possession of something according to Jewish law?
This is one of the central issues dealt with by Perek HaZahav, the fourth perek of Massekhet Bava Metzia. The first Mishna in the perek (44a) discusses the purchase of moveable objects, and specifically the status of money (i.e. coins) that is used to buy things.
Somewhat surprisingly, Jewish law does not recognize the validity of a kinyan – a purchase – made with money, unless the buyer also takes possession – at least symbolically – of the object that he purchased. This ruling is based on the opinion of Rabbi Yohanan who believes that on a biblical level purchases can be made with money, but the Sages limited that kinyan due to a fear that if the seller suffered a fire that destroyed part of his stock, he would claim that what was purchased had been destroyed (nisrefu hitekhah be-aliyah – “your wheat was burned up in the attic”).
Another acceptable method of kinyan is called halipin – exchange. Even though, as we have learned, transferring money to another person does not establish ownership of an object that has been purchased, if two people want to make an exchange, a kinyan would take place. For example, if two people agreed to exchange a cow for a donkey, once an act of kinyan is made on one of the animals, the second animal transfers to the possession of the other person with the kinyan made on the first animal.
Our Gemara discusses whether a coin can be used to purchase something by means of halipin. That is to say, can the buyer and seller agree that the kinyan should take place because the object will be exchanged for the money. The opinion that money cannot be used for halipin argues that when using money the point is the tzura – the image on the coin. Most rishonim understand that this means that a coin is not valued for the intrinsic worth of its metal, but for what it represents. Only an object with intrinsic value can be used for halipin.