As we learned on yesterday’s daf ordinarily we do not assume that a purchase – a kinyan – can be made with the exchange of money. Rabbi Yohanan teaches that although on a biblical level purchases can be made with money, the Sages limited that kinyan. There was a fear that if the seller suffered a fire that destroyed part of his stock after he was paid for certain items, that he would not make an effort to save the items as they no longer belonged to him and he would claim that what was purchased had been destroyed (nisrefu hitekhah be-aliyah – “your wheat was burned up in the attic”).
Furthermore, it appears that money cannot be used to effect purchases, even if it is done as part of an exchange – halipin – a method that works only when objects of value are exchanged.
The Gemara on our daf questions this assumption by quoting a Mishna that teaches that all moveable objects can purchase one another, to which Reish Lakish adds “even a bag full of coins can purchase a bag full of coins.” Rav Aha explains that this would only be true in a case where the coins had been removed from circulation – either by the monarchy or by the people no longer using them. Since the coins no longer had value as money, they could only be seen as valuable for the value of their metal, and as a commodity they are seen as ordinary objects, which can be purchased by means of halipin.
Occasionally the government would forbid use of certain coins, usually because of political reasons, e.g. when a rebellious leader would mint coins. The coins minted by bar Kozibah, more popularly known as bar Kokheva, leader of the great revolt against Rome, are one example. During the short-lived independent Jewish rule in Jerusalem at that time, bar Kokheva minted coins. As can be imagined, these coins that celebrated independence were forbidden to be used by the Roman government.
In addition, there were cases where local governments would diminish the value of coins in circulation by adulterating them with less expensive metals. Although the government promoted use of these new coins, residents refused to use them.