Can a Rov Who is Familiar With a Case Become a Dayan for That Din Torah?

Rav Dovid Grossman 

Question:

Two parties had a dispute and went to a Rov for mediation. Ultimately, he was not successful and the dispute is headed to a din Torah. Since this Rov is already familiar with the details of the case, is he allowed to be a Dayan in the din Torah? 

Answer: 

The Gemara in Sanhedrin (7B) derives from a pasuk that the arguments in a din Torah can only be heard in the presence of all parties. Dayanim may not listen to one person’s claims without the other party being present, and the parties themselves are forbidden from presenting it to the Dayanim. Therefore, if a Dayan speaks to one party first, [i.e. he was unaware that he would be asked to adjudicate], the other one can refuse to allow him to be a Dayan. If both sides are aware that he has already heard the case, and they still agree to allow him to be a Dayan, then it would be okay. 

I see very often that hearing one side of the story first can give a Dayan one view of events, but when he hears from the other side, he gets a whole different story. The first impression is very important, which is why hearing from one side first without the other being there is a very big problem.  

It often happens that the two parties have a Rov they trust and feel comfortable with and they want to try to negotiate through him before going to bais din. What I suggest is that they sign a shtar in which they agree to go to the Rov for mediation, and, if the negotiations through him alone fail, two other Dayanim will be added to create a bais din.