Gemara

Ploni came from Bei Chuzai and said that he is the brother of Mari bar Isak, and asked to divide their father's inheritance. Mari denied knowing him.

Rav Chisda: It is reasonable that Mari would not recognize his brother - "they (Yosef's brothers) did not recognize him" because Yosef left them before he had a beard, and now he had a beard. Ploni must bring witnesses that he is a brother in order to share the inheritance.

Ploni: I have witnesses, but they fear Mari, for he is intimidating!

Rav Chisda (to Mari): Go bring witnesses that he is not your brother.

Mari: Is that the law?! Ha'Motzi mi'Chavero Alav ha'Re'ayah (the one who seeks to take money from another, he must bring proof)!

Rav Chisda: This is the law for bullies.

Mari: If you are concerned that they fear me, even if witnesses testify for me, it will not help!

Rav Chisda: Witnesses would not transgress two matters due to fear.

Witnesses testified that Ploni is his brother. Ploni asked for half the vineyards and orchards, which Mari had improved. Rav Chisda agreed, for a Mishnah says that if minors and adults inherited, and the adults improved the property, the minors share in the improvements.

Objection (Abaye): There, the adults knew that they have minor brothers, and they improved the property knowing that this will be shared with the minors. Mari did not know that he had a brother. He improved for himself!

Bava Kama 112b (R. Yosi bar Chanina): If the claimant is sick, or if the witnesses must leave town, and the defendant was summonsed and did not come, we accept witnesses in his absence.

Rishonim

Rif: Rav Chisda told Mari to bring Ploni's witnesses.

Nimukei Yosef (DH Mari): Even without a reason why Mari would not recognize his brother, Ploni must bring proof! Perhaps because Mari was a bully and a swindler, he would not be believed without a reason. This is why Rav Chisda forced him to bring witnesses in the end. Do not say that if so, anyone can take advantage of bullies. There were Raglayim l'Davar (grounds to believe) that Ploni's claim was true and Mari was lying. It is not common that a stranger claims 'you are my brother', and that one does not know whether or not he has a brother. The end of the Sugya (Abaye) proves that Mari did not know that he had a brother. If Mari could not find witnesses that Ploni is not his brother, Ploni would get half the property.

Rambam (Hilchos Edus 3:12): If Beis Din knows that a defendant is intimidating and the claimant says that the witnesses will not testify because they fear him, Beis Din forces the defendant to bring the witnesses. In all such cases, this is how we deal with bullies.

Rosh (3:14): Ploni brought a proof that he had witnesses, but they fear Mari. When he told the witnesses to testify, they said 'why should we make trouble for ourselves?! We will not testify anything. Therefore, Rav Chisda told Mari to bring those witnesses to testify that Ploni is not his brother, of that they do not know him. He told Mari that this is the law for bullies because there were Raglayim l'Davar that they feared to testify. Witnesses would not transgress two matters due to fear. I.e. they would withhold testimony, but they would not lie.

Ritva (39b DH u'Farkinan): Yisre'elim are not suspected of false testimony! Rather, they would not transgress two matters, i.e. to withhold testimony, and later to say that they do not know.

Tosfos (39b DH Zil): Will we say that every rich bully must bring witnesses that one who claims from him is not his brother?! Here is different, for Ploni had witnesses who feared to testify.

Tosfos (Kesuvos 27b DH Omar): The Ri says that Ploni brought witnesses, and they said 'we will not tell you anything about this testimony.' This is Raglayim l'Davar that Mari is a bully. If there are no Raglayim l'Davar, one is not believed to say that his opponent is a bully. If not, everyone will claim this and say 'my opponent must bring witnesses, and if he does not, he must pay.'

Poskim

Shulchan Aruch (CM 28:5): If Beis Din knows that a defendant is a bully and the claimant says that the witnesses will not testify because they fear him (Rema - and there is a proof for his words), Beis Din forces the defendant to bring the witnesses. In all such cases, this is how we deal with bullies.

Beis Yosef (DH v'Im): Tosfos and the Rosh say that Rav Chisda told Mari to get Ploni's witnesses to testify that Ploni is not his brother, or that they do not know him. Rashi and the Nimukei Yosef say that he required Mari to bring Ploni's or other witnesses to testify that Ploni is not his brother. I.e. it does not suffice if they say that they do not know. Perhaps this is even if there are no Raglayim l'Davar. Do not say that if so, anyone can swindle. Beis Din must know that the defendant is established to be a bully. Perhaps the Rambam holds like this. He requires that Beis Din knows that the defendant is a bully, and the claimant says that the witnesses fear him. Do not say that if so, anyone can take advantage of bullies. This is how we deal with them until they repent and act like everyone else! The Nimukei Yosef connotes that we force the bully to bring proof only when there are Raglayim l'Davar that he is lying, and even if the witnesses do not say 'why should we make trouble for ourselves?' However, if there are no Raglayim l'Davar that he is lying, even if the witnesses say 'why should we make trouble for ourselves?', we do not obligate him to bring witnesses. The Tur rules like Tosfos, for the Rosh holds like Tosfos.

Lechem Mishneh (Hilchos Edus 3:12): One need not say that the Rambam holds like Rashi.

SMA (29): The Mechaber did not say 'there is a proof for his words.' He holds like the Rambam, that it suffices if Beis Din knows that he is a bully.

Taz: The Tur teaches that even he is established to be a bully, the claimant must bring a proof (that the witnesses fear him). However, a proof that the witnesses fear him suffices to establish him to be a bully.

Urim (22): If he proves that the witnesses are afraid to testify, this is reason to obligate the defendant to bring a proof, even if he is not a bully! The Rema requires a proof. He should have mentioned that the Beis Din need not know that he is a bully!

Pischei Teshuvah (17, citing Tiferes l'Moshe): The Tur connotes that he must be established to be a bully, and the claimant must proves that the witnesses fear him, but Tosfos holds that it suffices if he is established. It seems that when he is merely established, he just needs to get the witnesses to say that they do not know. If we know that the witnesses fear him, he must bring a proof.

Shulchan Aruch ((15): We do not accept testimony not in front of the litigant. If it was accepted, we do not judge according to it.

Rema: Some disagree and say that if it was accepted, b'Di'eved it is Kosher. Therefore, if a litigant is a bully and witnesses fear to testify in front of him, we accept their testimony not in front of him, and judge according to it. However, if it is possible to force the bully (to bring proof) like we said in Sa'if 5, this is better.

Terumas ha'Deshen (175): If witnesses fear a litigant, they should write their testimony for him. Ra'avan says that when there is Ones, testimony in the litigant's absence is Kosher l'Chatchilah. This includes when the litigant would use his power to delay acceptance of the testimony.

Question: Rav Chisda did not accept the witnesses in Mari's absence! A bully is not listed among cases in which we accept testimony not in front of the litigant!

Answer (Terumas ha'Deshen 176): The Gemara connotes that Mari knew who the witnesses were. They feared to testify even in his absence. If not, surely Rav Chisda would have told them to testify in Mari's absence, rather than annul 'ha'Motzi mi'Chavero Alav ha'Re'ayah.' The Pesul of testimony not in front of the litigant is lenient; we override it due to illnesss, witnesses who need to leave, and for validation of documents. Hefker Beis Din allows us to force an intimidating defendant to bring a proof or accept testimony in his absence. The Gemara taught when letter of the law, we accept testimony in the litigant's absence. The law for a bully is a fine. Also, it does not always apply, e.g. when witnesses fear to testify even in his absence.

Darchei Moshe (9): Rav Chisda did not accept the witnesses in Mari's absence. Perhaps they feared to testify in his absence. Alternatively, it is better to force the bully to bring a proof.