Kiddushin 74A

וניחזי זוזי ממאן נקט? לא צריכא דנקט מתרוייהו ואמר: חד מדעתאי, וחד בעל כורחי. ולא ידיע הי מדעתו והי לא מדעתו.

The Talmud asks: And let us see from whom he took the money, as it will be obvious that he is the one who bought it. The Talmud answers: No, it is necessary to teach this halakha in a case where the seller took money from both of them, and he then said: One payment I accepted willingly, and one payment was given to me against my will, and it is not known which person gave him money in accordance with his will and which did so against his will. In that case, if the item is no longer in the seller’s possession, he is not deemed credible to testify to whom he sold it.

נאמן דיין לומר: לזה זכיתי ולזה חייבתי. במה דברים אמורים ⁠– שבעלי דינים עומדים לפניו, אבל אין בעלי דינים עומדים לפניו ⁠– אינו נאמן. וניחזי זכותא מאן נקיט?

The Talmud cites the continuation of the baraita: Similarly, a judge is deemed credible to say: I found this person victorious in a civil case, and I found this one obligated to pay. In what case is this statement said? When the litigants are still standing before him. But if the litigants are not standing before him but have left, he is not deemed credible. The Talmud asks: And let us see who holds the writ of a favorable verdict. Why is there a need to rely on the statement of the judge?

לא צריכא, דקריע זכותייהו. וניהדר ונידיינינהו! בשודא דדייני.

The Talmud answers: No, it is necessary to teach this halakha in a case where their writs of a favorable verdict have been torn up and cannot be examined. The Talmud asks: If so, then let him return and judge them again, and presumably the same verdict will be issued. The Talmud answers: It was a case of the judges’ discretion [shuda dedayyanei]. In certain cases, the verdict depends on the decision of the judges based solely on their sense of which litigant deserves to win. There is no guarantee that they will make the same decision the second time around.