Gemara

Chachamim hold that anything a Shali'ach must ask about is not included in the species. R. Akiva holds that it is.

(Abaye): R. Akiva agrees that one is not lashed for eating things that a Shali'ach would ask about.

(Mishnah): If a Shali'ach (was sent to buy something with Hekdesh money, and he) fulfilled his mission, the sender transgresses Me'ilah. If he did not fulfill his mission (e.g. he bought the wrong item), he himself transgresses Me'ilah.

(Rav Chisda): This is unlike R. Akiva:

(Seifa): If a man told Levi 'give meat to the guests', and he gave liver (or vice-versa, and what he gave was Hekdesh), Levi transgressed Me'ilah.

A Shali'ach sent to buy meat would ask about liver. According to R. Akiva, he fulfilled the mission. The sender transgresses, and not the Shali'ach!

Rejection (Abaye): The Mishnah can be like R. Akiva. R. Akiva admits that the Shali'ach must consult! (Therefore, we do not say that he fulfilled his mission.)

The Chachamim who argue with R. Akiva hold like R. Shimon ben Gamliel:

(Beraisa): If one vowed from meat, all types of meat (e.g. cattle, flock and Chayos) are forbidden. He is forbidden the head, legs, windpipe, liver, heart, and fowl. He may eat fish and locusts;

R. Shimon ben Gamliel forbids all types of meat, but permits the head, legs, windpipe, liver, heart, and fowl, and all the more so, fish and locusts.

R. Shimon ben Gamliel says, innards are not considered meat. One who buys them in place of meat is not a (distinguished) person.

Question: Why does the first Tana forbid fowl, and permit fish? A Shali'ach would consult before buying either!

Answer #1 (Abaye): The case is, the Noder just underwent bloodletting.(He did not intend to eat fish anyway, for it is dangerous, so he didn't intend to forbid it.)

Question: Also fowl is dangerous to eat after letting blood!

Answer: He had fowl in mind, since if he overcooks it, it will not harm him.

Answer #2 (Rava): His eyes were hurting when he vowed.

Shmuel taught that fish are a remedy for the eyes. This is only at the end of the sickness. (Before this, fish are harmful.)

Chulin 103b (Mishnah): The only kinds of meat that one may cook with milk, or bring on the same table with cheese, are meat of fish and locusts.

If one vowed not to eat meat, he may eat meat of fish and locusts.

Question: The Reisha forbids cooking fowl with milk mid'Oraisa. It is unlike R. Akiva, who holds that the Isur of Chayos or birds with milk is only mid'Rabanan;

(Seifa): One who vows not to eat meat may eat meat of fish and locusts.

Inference: Fowl is forbidden. This is like R. Akiva, who says that anything a messenger would ask about is considered the same species!

Answer #1 (Rav Yosef): The Mishnah is like Rebbi. He holds like R. Akiva regarding vows, and like Chachamim regarding meat and milk.

Answer #2 (Rav Ashi): The entire Mishnah is like R. Akiva. The Reisha forbids cooking any kind of meat with milk. Some kinds are forbidden only mid'Rabanan. Only meat of fish and locusts is permitted even mid'Rabanan.

Rishonim

Rif and Rosh (Nedarim 18a and 7:1) bring the Beraisa, and say that the Chachamim who argue with R. Akiva hold like R. Shimon ben Gamliel.

Korban Nesan'el (3): Our text of the Rosh omits 'liver'. This is a mistake; it should be included. The Tana'im argue about liver in our Sugya and in Me'ilah. The Bach says that the Rosh's text of the Gemara omitted liver. This is wrong. If so, the Mishnah is Me'ilah would not be like either Tana!

Rif and Rosh (Chulin 37a and 8:2): The Seifa permits fish and locusts to one who vowed from meat. This implies that fowl and innards are forbidden. This is like R. Akiva, who forbids anything that a Shali'ach would ask about. Our Stam Mishnah here is like R. Akiva, so the Halachah follows him. Really, we do not know which Masechta was taught first. If Nedarim was taught after Chulin, the Stam Mishnah is followed by a Machlokes, and the Halachah does not (necessarily) follow the Stam. In any case, it is a Safek mid'Oraisa, so we must be stringent.

Ran (Nedarim 54b DH ul'Inyan): An individual (R. Shimon ben Gamliel) argues with R. Akiva. The Halachah follows R. Akiva against a colleague.

Rif and Rosh (ibid.): Therefore, only locusts are permitted. Fish are forbidden, for a Shali'ach would ask about them. The Mishnah permits fish only if the Noder let blood or his eyes hurt, in which case he did not plan to eat fish anyway.

Ran (DH v'Ha): The Ramban says that a Stam Neder includes only what is fitting for the Noder at the time, unless it is clearly included in what he said (but not what we include because a Shali'ach would ask about it).

Rambam (Hilchos Nedarim 9:6): Anything that a Shali'ach would ask about is included in the species. E.g. in a place where a Shali'ach sent to buy meat would ask 'I only found fish. (Should I buy it?)', an oath or vow not to eat meat forbids even fish. Anywhere, a vow to forbid meat forbids fowl and innards, but not locusts. If it appears that he vowed with intent only for beef, or only for beef and fowl, fish are permitted, even in a place where a Shali'ach would ask about fish.

Rebuttal (Ran ibid.): The Gemara equated fowl to fish. Both of them are forbidden only where a Shali'ach would ask about them! However, if the Noder let blood or his eyes hurt, fowl is forbidden and fish are permitted.

Defense #1 (Kesef Mishneh): The Rambam holds that a Shali'ach told to buy meat asks about both of them, but differently. He immediately asks whether he should buy fowl or beef, just like he asks whether he should get Behemos or Chayos. He asks about fish only later, if he did not find meat. And even if he would ask immediately, he would say 'if I find only fish...', not 'do you want fish?' The Gemara said what a Shali'ach would ask regarding fish, but not regarding fowl. It asked 'what is the difference between fowl and fish', even though they are different, for since a Shali'ach asks in both cases, both should be forbidden. The Gemara answered that the case is, the Noder let blood or his eyes hurt. It could have said that fish is different, but we wanted to establish the Chachamim of R. Shimon ben Gamliel to be like R. Akiva.

Defense #2 (Gra YD 217:11): In the days of the Gemara, in every place a Shali'ach would ask (about fish and fowl). This still applies to fowl, but nowadays a Shali'ach asks about fish only in some places.

Ran (ibid.): If one let blood, we permit fish only if he vowed only for that day. If he vowed for eight days or more, fish are included. Even though he did not need to forbid them today, he forbids them for the other days (when it is safe to eat them). If his eyes hurt, the Rashba permits fish even if he vowed for more than 30 days. He does not intend to forbid them, for he does not know when he will recover. I permit only if it is clear from his pain that he did not expect to eat them for the entire period.

Ran (DH u'Mistabra): Even if one vowed on the day he let blood, salted meat is forbidden. Even though he would not eat it that day, it is proper meat and a Shali'ach need not ask about it. Having let blood today is not a solid proof that he did not intend to forbid it. Bloodletting helps only for things that a Shali'ach would ask about. If it helped even for salted meat, the Mishnah would have taught this bigger Chidush!

Poskim

Shulchan Aruch (YD 217:8): In a place where a Shali'ach sent to buy meat would reply 'I found only fish', an oath or vow not to eat meat forbids even fish. Anywhere, a vow to forbid meat forbids fowl, innards, head, legs, windpipe, and heart, but not locusts. If it appears that he intended only for beef and/or fowl, fish are permitted, even in a place where a Shali'ach would ask about fish.

Taz (9): Nowadays one who vows from meat is permitted fish, for a Shali'ach would not ask about fish.

Rema: If he intended only for beef, even fowl is permitted.

Drishah (2): Nowadays, we eat fowl, i.e. chicken, after bloodletting! Why don't the Poskim permit fowl if one vowed after bloodletting? It seems that only Tziporim, i.e. wild birds, are dangerous, so he forbade chicken.

Rebuttal (Taz 10): We do not distinguish chickens from Tziporim. We have cooked chicken. Only roasted fowl is dangerous. The Poskim do not distinguish because in any case he forbids fowl, for he can eat it cooked.