Case: The plaintiff wanted to bring as a witness an agent who worked for the defendant. The defendant requested that beit din not allow the agent to testify because he is unfit to testify in this case for several reasons. Firstly, the agent is at odds with the defendant, and they are in the midst of adjudication. There is also a claim that the witness has been involved in criminal activity. He also is claimed to be a nogeiah badavar (one who has an interest in the outcome of the adjudication at hand), for if it is discovered that he acted without authority in this case, he may have to pay for that action.

Ruling:Despite the aforementioned, beit din decided to hear the witness’ testimony even if it were determined that it does not have the standing of kosher testimony. Beit din just has to be careful not to accept the testimony as direct grounds to decide the monetary issue.
The basis for this approach is the idea we see in the poskim that there is no prohibition to hear testimony from one who is not fit. The Shulchan Aruch (Choshen Mishpat 28:2) cites two opinions on whether one can force a relative to testify. The context is when one makes a cherem (ban) to make all witnesses testify, and the question is whether a relative or even a litigant must testify. Admittedly, the opinion that they are not required is the main one. However, it is apparent that the only question is whether they are required to come, but clearly if they step forward, there is no problem to listen to them. This is despite the fact that their testimony could not possibly be directly responsible for apportioning payment to one side or the other.
What, then, can be gained by the testimony? The Rambam (Sanhedrin 21:10) writes, in explaining that a dayan should not help a litigant, that when one witness testifies against a litigant, beit din should not say that beit din does not accept one witness [for payment]. Rather they should say: "The witness testified against you," with the hope that the litigant will admit that the witness testified accurately or not challenge him. The Haghot Oshri also describes the practice of having unfit witnesses testify as a special rule of batei din to uncover the truth (see Halacha Psuka (Eidut p. 54)).
Our experience shows that there is a lot to be gained from bringing such witnesses, as it helps greatly to uncover the truth. It forces the litigants to relate to details of the case in light of the testimony in a manner that beit din and the other litigant would not succeed to do without the testimony.
In this case, the witness actually confirmed much of the defendant’s, not the plaintiff’s, version of the story, and in many of the points, the plaintiff did not question that which was said.
In summary, whatever the exact benefit that can come out of it, beit din may allow the sides to call people to testify, even when they are halachically unfit, even though the testimony cannot be relied upon.