This Halacha is published in our Sefer “Topics in Practical Halacha” Volume 2
Hearing the claims of each side in the presence of the other side:
Monetary claims are a basic aspect of day to day business dealings and interaction with others. These claims must be brought before a Jewish court of law for their resolution and verdict. The Torah dictates a very basic principle that must be followed by both judges and plaintiffs, which is hearing both sides of the argument. It is forbidden for a judge to only hear one side. Furthermore, while hearing both sides of an argument is a basic ingredient for proper justice and fairness in a judgement, in addition, the Torah requires that the judge hear the claims of the plaintiff in the presence of the defendant, and it is forbidden for the judge to accept, or the plaintiff to state, his claim not in the defendant’s presence. This allows the defendant the fair chance to immediately deny any false claims, and prevents the first side of the argument from sinking into the mind of the judge for some time until the defendant is given a chance to say his side. Likewise, it intimidates the plaintiff from making a false or exaggerated claim, knowing that the defendant is right there to deny it. While this system is Halachically binding only towards a court of law, we can all derive from this a great lesson as parents and educators who often play the role of mediator in arguments amongst children and students, and that is not only to hear both sides of the argument, but not to hear the claim of one side until the other side is present. While not a Halachic requirement, doing so allows the parent/educator the chance to fairly hear about what happened and make a correct disciplinary decision. Often, we hear one side first and make a quick decision without ever giving the other child/student the chance to give a fair explanation of what happened. There is nothing more upsetting to a child or student to feel their side was not heard and they were unjustifiably punished.
B. The law:
It is forbidden for a judge to hear the claims of one side of the case [plaintiff or defendant] not in the presence of the other side. This prohibition applies equally for the plaintiff/defendant, that they may not make a claim in front of the judge unless the other person is present. [The Zohar states that one who hears one side without the other is considered as if he has accepted upon himself a foreign G-d.]
Bedieved: In the event that a judge transgressed, and heard one side of the argument not in the presence of the other, some Poskim rule he may still judge the case. Other Poskim, however, question this allowance.
If one does not plan on judging the case: If one does not plan on judging the case, then it is permitted for him to hear one side of the case not in the presence of the other. This applies even if there is a possibility that he may judge the case, but he does not know for certain that he will do so, he may nevertheless hear the claims of one not in front of the other, and then if he is asked to judge the case, he may do so with the consent of the other side, [after letting them know that he already heard the other side]. If, however, one does plan on being the judge in the case, then he may not initially hear the testimony.
If the other side consents, may the judge hear the claim not in the presence of the other?
If the both sides consent that the judge may hear the claim of one side without the other being present, then it seemingly is permitted to do so.
May a Rav hear a Shaala dealing with monetary claims from only one side?
The Rav [if he is not also the Dayan] may hear the Shaala and tell the person whether he may go to Beis Din for his claim. However, he is not to give any advice or ruling to the individual, until the other side is present, and he hears his claims as well.
 Michaber 17:5; Sanhedrin 7b; Shavuos 31a
 Biblical or Rabbinical: Some Poskim rule it is a Biblical prohibition for the judge to hear one side without the other being present. [Kneses Hagedola 17:13, brought in Pischeiy Teshuvah 17:8] See Pischeiy Teshuvah ibid
 The reason: The verse states “Ushimoa Bein Acheichem Ushifatitem”, which means that the judges must hear the claims of one side in front of the other side. [Smeh 17:10; Sanhedrin ibid] Alternatively, the reason is because the verse states “Lo Sisa Sheima Shav”, and when one side makes a claim not in the presence of the other, he has no shame to make dubious claims and statements. [Smeh ibid; Beir Hagoleh ibid; Rav Kahana in Sanhedrin ibid] This then leads to fear in the other side, that the fictitious claims that were aleady heard were absorbed in the mind of the judge as true. [Smeh 17:11; Aruch Hashulchan 17:7]
 The reason: As the verse states “Lo Sisa Sheima Shav” and can be read as “Lo Sasi.” [Smeh 17:12; Sanhedrin ibid]
 Zohar Vayeishev p. 149, brought in Pischeiy Teshuvah ibid; Aruch Hashuclhan 17:7
 Shach 17:9; Implication of Smeh 17:11; Poskim brought in Kneses Hagedola 17:13 and Pischeiy Teshuvah 17:8; Vetzaruch Iyun from Rama ibid who implies that even Bedieved one may only hear the case if the defendant agrees, and so rules Bach 17
 Kneses Hagedola ibid
 Rama 17:5; Smeh 17:11
 May one advise one of the person’s in the case as to the Psak? One may not give a potential ruling to the case until he hears both sides, even if he is not the judge and his rulings are not binding. [Rama ibid] Thus, while he may hear one side not in front of the other, if he does not plan on judging the case, he may not tell the person a ruling regarding it.
 Smeh 17:11
 The reason: As perhaps the other side does not feel comfortable being judged by a person who already heard the claim against him. [Smeh ibid]
 Clear implication of Rama and Smeh ibid; See Aruch Hashulchan ibid
 Rama ibid