1,197. A Self-Appointed Advocate

181:15 If a person would benefit from his testimony, or if the witness has any interest however remote in the case, then he is unfit to testify.

181:16 Ezekiel 18:18 says, "that which is not good among his people." The Sages (Shevuos 31a) explained that this refers to one who acts like an attorney and gets himself involved in a dispute not his own. This is meant specifically when the two litigants are both in the town but the borrower is difficult and has many arguments so the lender is scared to argue against him and appoints a representative. This is what's meant by one who gets involved in a dispute not his own. However, if the defendant is in another city and the plaintiff cannot go, so he appoints a representative, then the one so appointed has a positive commandment to defend the injured party from the one who wronged him. Some authorities say that one may be appointed if he is doing it for pay and not for just to show off his prowess.