Our Gemara quotes a Mishna that rules that if someone slaps his friend, and he remains uninjured, he is obligated to pay him a sela because of the embarrassment that he caused.
When such a case was brought before Rav Yosef, he ruled that it was to be paid from kesef medina – a coin of lesser value. Upon hearing this, the man who had been slapped announced that he had no desire to accept such a small payment, and that he preferred that it be given to charity. Afterwards he changed his mind and requested that the money be given to him. Rav Yosef responded that he could no longer change his mind, since the beit din,which represented the poor, had already taken it on their behalf.
The rishonim ask why it was necessary for Rav Yosef to give this ruling – in any case the man had accepted upon himself to distribute money to the poor and would be obligated to do so. The Ri”f and Tosafot say that his original statement did not obligate him, since he was promising money that did not yet belong to him. Rabbeinu Hananel suggests that the man was not reneging on his original promise, he simply changed his mind about giving that specific coin to the poor – he planned to give other money instead. Rav Yosef told him that he could not make even that small change.
Generally speaking, during the time of the Talmud there were two types of coins. Matbe’ah tzuri was a silver-based coin that was viewed as being biblical money. Kesef medina were coins that had the same names as the more valuable matbe’ah tzuri, but were made of cheaper metals and were worth 1/8 the value of kesef tzuri. Different values for coins with identical names were not uncommon in the ancient world and this phenomenon still exists in some places today, where paper money may have the same name as a gold coin, for example, but is worth significantly less. It is therefore essential to determine which coin is being discussed.