Gemara

(Mishnah): If a Nazir became Safek Tamei (Mes), and he was a Safek Metzora Muchlat, he shaves four times (twice for (Safek) Tzara'as, for (Safek) Nazir Tamei, and for Nazir Tahor).

Question: Why may he shave four times? He transgresses Hakafah (cutting the sideburns) if he was not a Muchlat or not Tamei Mes! Some texts - he transgresses shaving the beard.)

Answer (Shmuel): The case is, the Nazir is a woman or child. (They may shave the beard and sideburns.)

(Rav Huna): One who is Makif a child is liable.

He equates "do not be Makif the corners of your head" and "do not destroy the corners of your beard." Whoever is commanded about the beard is commanded about Hakafah. Since women are not commanded about the beard, they are not commanded about Hakafah.

Rav Ada holds that the verse equates the Makif (who cuts Pe'os) and the Nikaf (whose Pe'os are cut). Whenever the Nikaf is liable, the Makif is liable. The Torah does not punish children. Since a child is exempt, one who is Makif a child is exempt.

Toras Kohanim (Metzora 1:3): "Toras ha'Metzora...ha'Kohen" teaches that a Kohen must slaughter the bird, sprinkle its blood and shave the Metzora.

Rishonim

Rif and Rosh (Makos 4a and 3:3): The Halachah follows Rav Huna. If a woman was Makif or Nikaf, she is exempt.

Rambam (Hilchos Tzara'as 11:3): A Kohen must shave a Metzora both times.

Minchas Chinuch (373 DH v'Lav): It seems that since the Torah required a Kohen to shave the Metzora, the Kohen must be an adult. The Sha'ar ha'Melech (Avodah Zarah 12:1 Sof DH Ra'isi b'Michtav) does not require an adult. He says that the Aseh of Tiglachas is Docheh the Lav and Aseh of a Nazir, also for the Kohen, and it must be done during the day.

Rambam (Hilchos Avodah Zarah 12:1): A Nikaf is not lashed unless he helped the Makif. One who shaves a minor is lashed.

Ra'avad: Even though a Nikaf is not lashed, since he consented, he transgresses a Lav.

Question: Rav Ada equates the Makif to the Nikaf (when the Nikaf is exempt, also the Makif is). He should likewise equate a Nikaf to a Makif, and permit a man to be Nikaf by a Nochri!

Answer (Tosfos 57b DH v'Rav): The verse primarily obligates the Nikaf. Therefore, it is reasonable to exempt the Makif when the Nikaf is exempt. It is unreasonable to exempt the Nikaf just because the Makif is exempt. If we would not say so, why does Tiglachas Metzora override Hakafah (Yevamos 5a),? We should require a Nochri, woman or child to shave the Metzora!

Tosfos (Shavu'os 3a DH v'Al): We could say that Tiglachas Metzora overrides Hakafah when a Nochri, woman or child is not available. We do not wait for one, just like the Gemara (Chulin 141a) suggested that the Aseh of Taharos Metzora overrides Shilu'ach ha'Kan. We do not say that he should wait for another bird! However, we can prove this from Shmuel, who said that the Safek Metzora who is shaved must be a woman or child. Shmuel must hold like Rav Ada, since he permits shaving a woman or child (mi'Safek). If Rav Ada permitted a man to be shaved by a Nochri, woman or child, Shmuel could have said that this is how the Metzora is shaved mi'Safek!

Maharsha (on Tosfos): Tosfos holds that Rav Ada obligates the Makif whenever the Nikaf is liable, i.e. when Hakafah applies to the Makif.

Rebuttal (Sha'ar ha'Melech Avodah Zarah 12:11 DH v'Ra'isi): In Nazir, Tosfos says that one who is shaved by a woman is liable! Rather, Tosfos here means when a Nochri or child is not available. 'Woman' was included Agav; Dichuy would be needed even when she is Makif.

Afikei Yam (2:34): Some learn from Tosfos that there is no Mitzvah to shave a Metzora; rather, the only Mitzvah is that he be shaved. If it was a Mitzvah, surely we could not rely on a child or Nochri! However, the Torah implies that it is a Mitzvah, and a Kohen must do it. The Rambam rules like this in Hilchos Tzara'as (11:3). The Sha'ar ha'Melech says that Tosfos concluded that our Sugya disagrees with the Toras Kohanim. Besamim Rosh (18) suggests that l'Chatchilah a Kohen is required, but this is not Me'akev. In any case, it is a Mitzvah to shave him.

Question: Tosfos holds that since he becomes Tahor no matter who shaved him, if it could be done without Dichuy, it would not be Docheh. If so, we can learn from there! Also, even in the conclusion that Rav Ada agrees that the Nikaf is always liable, why may a man shave him? We should require someone who is not commanded, to minimize the Dichuy!

Answer (Afikei Yam DH Ulam): Reish Lakish says that when it is possible to fulfill both, there is no Dichuy. This is only if the person himself can fulfill both. If he cannot, there is Dichuy, even if someone else could fulfill both without Dichuy.

Or Some'ach: Korban Aharon says that since Tiglachas could be done later, it is considered possible to fulfill both. This is wrong. The Gemara (Shabbos 133a) teaches that Milah after day eight does not override Yom Tov. The Rashba says that delaying Milah is not considered fulfilling both.

Question: An Aseh is Docheh a Lav only if one fulfills the Aseh at the time he transgresses the Lav. A Metzora fulfills Tiglachas only when he finishes, but he transgresses once he starts cutting one Pe'ah!

Answer #1 (Tosfos Gurnish, brought in Aruch l'Ner Yevamos 5a v'Hinei): Indeed, it is permitted only if one Pe'ah was already shaved.

Rebuttal (and Answer #2 - Aruch l'Ner): Even then, the Mitzvah is only when the entire Pe'ah is shaved, and he already transgressed! Rather, the Nimukei Yosef (Bava Metzia 16a) says that as long as one is engaged in the Mitzvah, it is Docheh, even if the Mitzvah will not be finished until later.