The Torah provides certain scenarios that a beis din (court) is required to hear. The Torah also tells us that the beis din must try any case in which one person sues another. This can include any number of situations, including unpaid debts, theft, withheld wages and more. Basically, any kind of case where one person says, “Did not!” and the other says, “Did too!” This is called the law of the to’ein and the nitan, the plaintiff and the defendant.
The Torah says that the judges must hear any kind of case where one person claims money or property from another where he can say, “This is it!” The Talmud in Baba Kama (107a) tells us that even though that particular phrase is written in the section dealing with an unpaid watchman (see Mitzvah #57), it applies elsewhere as well, such as to the case of loans.
The rules vary based on the testimony of the defendant. If the plaintiff claims the defendant owes him, say, $200 from an unpaid loan, how does the defendant react? He could conceivably reply, “Yes, I borrowed $200, but I paid you back,” “Yes, I borrowed from you, but it was only $100,” or “What are you talking about? I never borrowed any money from you!” The defense then affects the course the trial will take, including whether or not the court will require the defendant to swear as to the content of his testimony.
The Sefer HaChinuch lists many, many, many details about the way this mitzvah is carried out. Among these are such details as:
* that testimony is only accepted when all the litigants are present;
* what to do if the lender wants to be repaid in the presence of certain witnesses, who are unavailable;
* that the repayment of a loan can be reassigned to a third party if all three parties are present;
* the reliability of a shopkeeper’s ledger;
* in what cases one is believed if he says he was only kidding;
and many, many more. See there for a comprehensive overview.
As with other court matters, this mitzvah applies to the judges, who are obligated to try these cases according to the Torah’s laws. This mitzvah is discussed in the Talmud in the tractates of Baba Kama (chapter 3), Baba Metzia (chapter 1), Baba Basra (chapter 8) and Shevuos (chapters 5-7). In the Shulchan Aruch, it is codified in Choshen Mishpat 110. It is #246 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos.