Bava Kamma 75A

שלא בבית דין הוה קאי.

The Talmud answers: Nevertheless, R' Yehoshua was not present in court when Rabban Gamliel encountered him.

והתניא, אמר לו: אין בדבריך כלום, שכבר הודית,

The Talmud asks a question from a different source: But isn’t it taught in a baraita that R' Yehoshua said to Rabban Gamliel: Your statement is nothing, as you have already admitted to inflicting the injury yourself? This indicates that even if witnesses would subsequently testify about the injury, Rabban Gamliel would not emancipate Tavi.

מאי, לאו תנאי היא ⁠– האי תנא דאמר ״שכבר אין לך עדים״, סבר: מודה בקנס ואחר כך באו עדים ⁠– חייב; והאי תנא דאמר ״שכבר הודית״, סבר: מודה בקנס ואחר כך באו עדים ⁠– פטור?

The Talmud suggests: What, is it not the case that the difference between these two baraitot is a dispute between tanna’im? This tanna of the first baraita, who says that R' Yehoshua’s statement was: As you have no witnesses, holds that one who admits that he is liable to pay a fine is liable to pay the fine if afterward witnesses come and testify to his liability. And that tanna of the second baraita, who says that R' Yehoshua’s statement was: As you have already admitted, holds that one who admits that he is liable to pay a fine is exempt from payment, even if afterward witnesses come and testify to his liability.

לא; דכולי עלמא, מודה בקנס ואחר כך באו עדים ⁠– פטור; ובהא קמיפלגי ⁠– האי תנא דאמר ״שכבר אין לך עדים״, סבר: חוץ לבית דין הוה. והך תנא דאמר ״שכבר הודית״, סבר: בבית דין הוה.

The Talmud rejects this suggestion: No; it is possible to understand the baraitot differently. Everyone agrees that one who admits he is liable to pay a fine is exempt, even if afterward witnesses come and testify to his liability. And they disagree with regard to the following: This tanna, who says that R' Yehoshua said: As you have no witnesses, holds that R' Yehoshua was outside the court when Rabban Gamliel encountered him, and therefore his admission is disregarded. And that tanna, who says that R' Yehoshua said: As you have already admitted, holds that R' Yehoshua was in the court when Rabban Gamliel met him, so Rabban Gamliel’s admission is a valid admission.