Q: Can a person with a vested interest in the case testify in beis din?
A: A witness who has a monetary interest in a case, even indirectly (nogei’a badavar), is disqualified from testifying. The determination of indirect interest depends on the Dayan’s evaluation. Most authorities explain the disqualification as concern that the witness might lie, or distort the truth, on account of his personal interest (C.M 37:1,21; SM”A 37:1).
Therefore, such a witness is permitted to testify to his detriment. Additionally, if he dissociates himself from any monetary connection, he may now testify, even if he witnessed the incident while he had an interest. However, if the witness signed a document when he had an interest, the document remains invalid even after he dissociates himself (C.M. 33:15; Pischei Teshuvah 33:9).
If the witness has no interest at the moment, but might have in the future, whether he is qualified depends mostly on the likelihood. (See C.M. 37:10; Ketzos 37:5; Pischei Choshen, Eidus 2:36 .)