Nedarim 54B

מי לא מודה רבי עקיבא דצריך אימלוכי? איתמר שמעתא קמיה דרבא. אמר להון: שפיר אמר נחמני.

doesn’t R' Akiva concede that the agent must consult his employer? Because he failed to do so and acted on his own, he is not considered to have performed his mission. This halakha was stated before Rava. He said to those who stated the halakha before him: Naḥmani, i.e., Abaye, spoke well.

מאן תנא דפליג עליה דרבי עקיבא ⁠— רבן שמעון בן גמליאל היא. דתניא: הנודר מן הבשר ⁠— אסור בכל מיני בשר, ואסור בראש וברגלים ובקנה ובכבד ובלב, ובעופות, ומותר בבשר דגים וחגבים.

§ The Talmud asks: Who is the tanna who disagrees with the opinion of R' Akiva in the Mishnah here? The Talmud answers: It is Rabban Shimon ben Gamliel, as it is taught in a dispute in the baraita: For one who vows that meat is forbidden to him, it is prohibited to eat all types of meat, and it is prohibited for him to eat meat of the head, and of the feet, and of the windpipe, and of the liver, and of the heart, although people do not typically eat meat from those parts of the body. And it is prohibited for him to eat meat of birds, as it too is popularly called meat. However, it is permitted for him to eat of the meat of fish and grasshoppers, as their flesh is not called meat.

רבן שמעון בן גמליאל אומר: הנודר מן הבשר ⁠— אסור בכל מיני בשר, ומותר בראש וברגלים ובקנה ובכבד ובלב ובעופות, ואין צריך לומר בשר דגים וחגבים. וכן היה רבן שמעון בן גמליאל אומר: קרביים לאו בשר, ואוכליהן לאו בר אינש. אוכליהן כבשר, לענין זביני ⁠— לאו בר אינש.

Rabban Shimon ben Gamliel says: One who vows that meat is forbidden to him, is forbidden in all types of meat, and is permitted to eat meat of the head, and of the feet, and of the windpipe, and of the liver, and of the heart and of birds, and needless to say he may also partake of fish and grasshoppers. And Rabban Shimon ben Gamliel would likewise say: Innards are not considered meat, and one who eats them is not a person, meaning that the innards are not fit for human consumption. The Talmud elaborates: With regard to one who eats them, in terms of the halakhot related to their consumption, e.g., vows, they are considered as meat. However, with regard to purchase, one who purchases them is not a person. In any case, apparently, Rabban Shimon ben Gamliel disagrees with R' Akiva, as he maintains that although if an agent fails to find meat he is required to consult his employer before replacing it with liver, it is not considered meat with regard to vows.