Musar על בבא קמא 198:25

The Talmud illustrates this point by various examples. In Baba Kama 99b we are told that a woman came to Rabbi Chiyah asking him to determine if a certain coin was sound. He told her that it was. The next day the same woman came back to him complaining that others had refused to accept the coin at full value. Thereupon Rabbi Chiyah instructed Rav, the keeper of his money, to give this woman a coin of his own to compensate her for what he perceived to be an error of judgment on his part. He asked Rav to record this transaction as a loss. The Talmud questions why this instance was different from the previously mentioned example of a professional money-changer who erred in his judgment and who had given his opinion without charging. Such a money-changer does not have to compensate the enquirer. The Talmud answers that Rabbi Chiyah did not have to make restitution, but that he acted לפנים משורת הדין, beyond what was demanded of him legally. He took his cue from Rav Yossi who interpreted Exodus 18,20: "You will inform them of the path they should take and the practices they are to follow" to mean that one should go beyond the demands of the law in dealings with fellow human beings.