Bava Metzia 44: Commodities and Currency - Which is Which?

Commenting on our last Mishna, the Gemara explains why a bailee is liable for a full barrel of wine in his charge if he lifts it to drain a quarter log and it breaks, where he is only liable for a quarter-log of wine if he drains that much from the barrel tilted on its edge and it breaks.  The rabbis note that the wine would ferment in the former case but not in the latter case.

Moving on to Perek IV, we are introduced to a new Mishna.  Using the example of gold, silver and copper coins in a number of scenarios, it teaches a new principle: When two people exchange a commodity for money, pulling the item denotes the exchange rather that the transfer of money from one to the other.  Rabbi Shimon notes that the person who holds the money has the advantage in an exchange, for he can reneg on the agreement until the commodity has been pulled.  

The Gemara begins by clarifying these statements made by Rabbi Yehuda HaNasi in his youth; these are said differently in his older years.  The rabbis understand the principle as constant: when one is purchasing coins with other coins, it is not important whether the gold, silver or copper coins are most valuable at one particular time and place.  What is important is that one party holds the currency and the other holds the commodity.  

The rabbis wonder how the fluctuating value of a commodity should influence the exchange.  If gold coins have increased in value by the time that they are returned, for example, does a borrower give an additional gold coin to the owner to account for their rise in value?  The rabbis do not want to put a person in the position of paying interest to a Jewish lender, which is not permitted. 

After giving a short thought to the value of a peruta versus a gold coin in the payment of a ketuba, the Gemara ends today's daf in a conversation about the redemption of second tithe produce.  When we are considering the changing value of coins, can these coins be used to desacralize second tithe produce?  This is the ten percent of a person's produce remaining after first tithe has been separated.   Second tithe is brought to Jerusalem and eaten there. Or, if it is too difficult to transport, second tithe produce can be sold and those coins can be used to buy food that is to be eaten while one is in Jerusalem.  The rabbis determine that coins should not be desacralized by second tithe produce.  However, if this has been done, the produce must be eaten in Jerusalem.

The rabbis attempt to understand whether gold is always a currency or whether it is sometimes a commodity.  A challenge in today's world, as well.