Ketubot 20A

וכשם שאין מזימין את העדים אלא בפניהם, כך אין מכחישין את העדים אלא בפניהם.

And just as witnesses render other witnesses false, conspiring witnesses only in their presence, because with their testimony they render them liable to be punished, so too, witnesses contradict the testimony of other witnesses only in their presence. Since the signatories to the document are dead, their testimony cannot be contradicted.

אמר ליה רב נחמן: אילו הוו קמן ומכחישין להו, הוה הכחשה, ולא הוה משגיחין בהו. דהוי[א] לה עדות מוכחשת. השתא דליתנהו, דאילו הוו לקמן דלמא הוו מודו להו, מהימני?!

Rav Naḥman said to Rav Sheshet: If the first pair of witnesses was before us and the second pair would contradict their testimony, that is contradiction, and we would not consider their testimony and would not collect money with the document, as it is contradicted testimony. Now that they are not before us, and in a case where if they were before us perhaps they would have admitted to the second witnesses that the testimony of the second witnesses is correct, are they deemed credible, and the document that they signed valid?

אלא אמר רב נחמן: אוקי תרי להדי תרי, ואוקי ממונא בחזקת מריה, מידי דהוה אנכסי דבר שטיא. דבר שטיא זבין נכסי. אתו בי תרי, אמרי: כשהוא שוטה זבין, ואתו בי תרי ואמרי: כשהוא חלים זבין.

Rather, Rav Naḥman said in the case where the testimony of the first witnesses is contradicted not in their presence, the ruling is: Establish two witnesses against the two witnesses who contradict their testimony, thereby neutralizing both testimonies, and establish the money in the possession of its owner, just as it was in the case of the property of bar Shatya. As when bar Shatya, a man who suffered from periodic bouts of insanity, sold his property, two witnesses came and said: He sold it when he was insane; and two other witnesses came and said: He sold it when he was sane.

אמר רב אשי: אוקי תרי להדי תרי, ואוקי ממונא בחזקת בר שטיא. ולא אמרן אלא דאית ליה חזקה דאבהתיה. אבל לית ליה חזקה דאבהתיה, אמרינן: כשהוא שוטה זבן, וכשהוא שוטה זבין.

Rav Ashi said in that case: Establish two witnesses against the two witnesses who contradict the testimony of the first pair, and establish the money in the possession of bar Shatya. The Talmud notes: We say that the property remains in the possession of bar Shatya only when he has possession of the property based on the possession of his fathers. However, if he does not have possession of the property based on the possession of his fathers, but he acquired the property himself, we say: He purchased his properties when he was insane, and he sold them when he was insane. He does not have presumptive ownership of them. Therefore, the property remains in the possession of the person to whom bar Shatya sold it.