There is at least one chronicle account of this duel, by Jean Juvénal des Ursins:
Michaud and Poujoulat (eds.), Nouvelle collection des mémoires pour servir à l'histoire de France, prémiere serie, vol. II pp. 371, 372 wrote:
En Bretagne audit temps avoit un chevalier nommé messire Robert de Beaumanoir, qui fit appeller devant la duc un autre chevalier nommé Pierre de Tournemine, en gage de bataille. Et disoit qu'il avoit un sien parent de son nom et armes, lequel on chargeoit d'entretenir la fille d'un laboureur, devers lequel vint ledit de Tournemine, et luy dit, qu'il estoit bien meschant, qu'il ne tuoit, ou faisoit mourir le parent dudit de Beaumanoir, veu la cause dessus dite, et luy conseilloit qu'il le fist; et tellement il enhorta ledit laboureur, qu'il se mit en aguet de la tuer par plusieurs fois, et le trouva une fois à son advantage, et le tua. Et disoit ledit de Beaumanoir, que le meurtre avoit esté fait [p. 372 starts here] par l'induction dudit Tournemine, et que faussement et mauvaisement il l'avoit fait; et s'il le vouloit nier, il estoit prest de l'en combatre, et jetta son gage. Tournemine respondit, en niant tout ce que disoit Beaumanoir. Et finalement veue la matière, et tout consideré, le gage fut adjugé, et dit qu'il y avoit gage de bataille. Et y eut jour et lieu assigné, auquel les parties comparurent en la presence du duc, et furent les sermons faits en la maniere accoustumée. Et aprés cry fait, que chacun fist son devoir, ils s'approcherent l'un de l'autre, et combatirent bien longuement, et ne sçavoit-on à peine lequel avoit le meilleur; et finalement de Tournemine fut desconfit, sans recognoistre le cas, et comme mort fut mis hors du champ.
In Britany at that time (the year 1386) there was a knight named Monsieur Robert de Beaumanoir, who called before the duke another knight named Pierre de Tournemine, on pledge (gage) of battle. And he said that he had a relative of his name and arms, who was obliged to maintain the daughter of a worker (laboureur), whom the aforesaid de Tournemine approached and told that he would be a real no-good, if he did not kill or put to death the relative of the said de Beaumanoir, in view of the cause mentioned above, and advised him that he could do it; and so he encouraged the aforesaid worker, who got ready to kill him on many occasions, and one day he found him at a disadvantage, and killed him. And the aforesaid de Beaumanoir said that the killing had been done on the encouragement of the aforesaid Tournemine, and that he had done this falsely and wickedly, and if he wanted to deny this, he was ready to fight, and threw down his pledge. Tournemine replied by denying everything which Beaumanoir had said. And in the end, in view of the situation, and all things considered, the gage was approved, and he said that it was a pledge of battle. And the chosen day and place arrived, on which the parties appeared in the presence of the duke, and speeches were given in the customary fashion. And after the cry had been given, that each should do his duty, they approached each other, and fought for a really long time, and we hardly know which of the two had the better. In the end de Tournemine was overcome, without retracting the case, and was brought out of the field as if he were dead.
I found the reference in Steve Muhlberger / Duke Finnvarr,
Deeds of Arms, p. 50. I may edit this post to include
the original French.