Reference for Bava Metzia 87:13
רבי שמעון אומר כל שהכסף בידו ידו על העליונה:
HE WILL TAKE VENGEANCE OF HIM WHO DOES NOT STAND BY HIS WORD. R. SIMEON SAID: HE WHO HAS THE MONEY IN HIS HAND HAS THE ADVANTAGE.<span class="x" onmousemove="('comment',' Lit., 'his hand is uppermost'. The general principle of this Mishnah is this: When one makes a purchase, the delivery of the money does not complete the transaction, and either party can withdraw from the bargain; on the other hand, once the goods are taken, the transaction is absolute and irrevocable, and neither party can withdraw, the purchase price being regarded henceforth as an ordinary debt caused by a loan. Now, in ancient days. when the value of coins depended on their weight and general condition, coins of one metal or denomination might themselves be purchased with other coins. Consequently, in such a transaction, it becomes necessary to determine which is to be regarded as the money and which as the goods. The Mishnah proceeds on the principle that those coins which have greater currency than others rank as money vis a vis the others, which are then regarded merely as movables. Now, silver coin had greater currency than gold coin — probably because the latter represented an unusually large sum of money in an agricultural community where money is generally scarce. Consequently, if one purchase gold denarii for silver denarii, as soon as he takes possession of the gold, the bargain is irrevocable and he is bound to render the silver coins to the vendor, i.e., the gold of the vendor gives him a legal title to the silver. On the other hand, if he first takes possession of the silver, the bargain is not concluded; hence revocable. On the same lines, copper coin rank as money vis a vis silver, so that when the former is taken, the transaction is legally closed; but not the reverse. The same principle operates in the other clauses of the Mishnah dealing with the purchase of money. In the case of barter, however, as soon as one party takes possession of the article that is bartered, the transaction is consummated, and neither party may withdraw. ');"><sup>12</sup></span>