“One witness may not arise against a man for any sin or iniquity!” (Devarim 19:15)
It is a negative commandment that we should not punish or extract money on the word of only one witness. This is true even if the witness is at the pinnacle of correct behavior and wisdom, and even if he is a prophet, and about this it says “One witness may not arise against a man for any sin or iniquity!”.
Amongst the roots of this commandment are that since the inclination of the heart of a person is evil, and sometimes a man will be upset with his friend, and even if he is at the top level of correct behavior, he will still not always be free of sin, and even if a person manages to stand for many days on the straight path it is not impossible for him to change his thoughts and to become evil, as we find that Chazal said that Yochanan the High Priest served as a High Priest for many years and at the end he became a Tzeduki (a type of heretic) it follows that it is correct and fitting that one should not rely on the word of one person to punish someone else even if the witness is a great and wise man and the criminal is an evil and ignorant man. However, when there are two good witnesses, there is a Chazakah (assumption) about all of the nation of Yisrael that they will not agree to testify falsely, and such assumptions have great legal strength in all things.
Amongst the laws of this commandment are that even though a single witness cannot make someone pay money, he can obligate someone to swear that he does not owe the money, and it follows that if Reuven claims that Shimon owes him money, and Shimon denies it, and Levi testifies by himself that Shimon does actually owe the money, Shimon is obligated to swear that he does not owe the money, and if he does not swear, he must pay. Another law is that even though a single witness is not believed for punishments, he is believed for prohibitions. It follows that if a single witness testified that meat or wine is Kosher or not Kosher, that he is believed.
This commandment applies to monetary laws which are relevant today, in all places and at all times, to men as it is upon them that it is incumbent to mete out justice.