The Mishna (98b) teaches that a craftsman is responsible for damage that he does to an object that was given to him to fix or to build. This ruling leads to a general discussion of the level of responsibility that a person has with regard to professional advice or services that he performs.
According to the Gemara, under ordinary circumstances, if someone pays an expert money-changer for professional advice in establishing whether a coin is valid or not, and the money-changer was mistaken in rendering his opinion, he will have to pay damages. An exception to this would be people who are complete experts and are recognized as knowing all that there is to know about the value of coins. If someone like that erred because of a change of currency (not because of a problem with the coin itself), he would not be responsible.
The Gemara tells of a woman who approached Rabbi Ḥiyya and asked him to offer his opinion on the validity of a coin. Rabbi Ḥiyya ruled that the coin was a good one, but the woman returned the next day and told him that it was not accepted in the market. Rabbi Ḥiyya instructed that the woman be paid the value of the coin, and that it should be recorded in his pinekas – his record book – as an undeserved loss. The Gemara explains that although Rabbi Ḥiyya was a high level expert, he chose to pay the woman “Lifnim mi-shurat ha-din – beyond the letter of the law”.