Bava Batra 128A

אתה נתתו לי במתנה. רצונך השבע וטול?״ ונשבע ⁠– אינו יכול לחזור בו.

or: You gave him to me as a gift, if he then adds: Although I owe you nothing, as the slave is mine, if it is your desire, take an oath that he is yours and take him; and the claimant takes an oath that the slave is his, the one in possession of the slave cannot retract his offer, although he was not obligated to make the offer.

מאי קא משמע לן? תנינא, אמר לו: ״נאמן עלי אבא״; ״נאמן עלי אביך״; ״נאמנין עלי שלשה רועי בקר״⁠ – רבי מאיר אומר: יכול לחזור בו, וחכמים אומרים: אינו יכול לחזור בו!

The Talmud asks: What is R' Abba teaching us? We already learned in a Mishnah (Sanhedrin 24a) that if one of the litigants said to the other: My father is trusted to adjudicate for me, or: Your father is trusted to adjudicate for me, or: These 3 cattle herders who are not experts in halakha are trusted to adjudicate for me, and all of the individuals this litigant mentioned are legally disqualified from serving as judges, R' Meir says that the one who stated this can retract his statement, and the Rabbis say that he cannot retract it, and he is obligated to have them serve as judges if the other litigant so desires. Since this halakha is already stated in the Mishnah, what novel halakha did R' Abba teach?

הא קא משמע לן ⁠– דב״אתן לך״ מחלוקת, והלכה כדברי חכמים.

The Talmud in tractate Sanhedrin records a dispute between amora’im as to the circumstances of the case of that Mishnah, with one amora holding that the dispute pertains only to a case where the one accepting the disqualified judge will need to forgive a debt if the judge rules in accordance with his opponent, while another amora holds that the dispute pertains even to a case where he will need to pay the other litigant if the judge rules in accordance with his opponent. By stating his ruling in a case where the litigant will have to give the slave to the other, R' Abba teaches us this, that the dispute between R' Meir and the Rabbis is even in a case where the litigant said: I will give you what you claim if that is the ruling of these judges, and the Rabbis hold that if he subsequently wishes to retract his suggestion, he cannot do so. And R' Abba also teaches that the halakha is in accordance with the statement of the Rabbis.