In Mishneh Torah Laws of Kings and their Wars 1:6 it states: ואין מעמידין מלך ולא כהן גדול. לא קצב. ולא ספר. ולא בלן [א.] ולא ברסי. לא מפני שהן פסולין אלא הואיל ואמנותן נקלה העם מזלזלין בהן לעולם. ומשיעשה במלאכה מאלו יום אחד נפסל: A butcher, barber, bath-attendant or tanner may not be chosen either as a king or as a Kohen Godol (High Priest). Not because they are disqualified, but because these professions are degrading they are always viewed trivially in the eyes of the people. Even if he worked at one of these vocations for but one day, he is disqualified (from becoming king). If someone for example gave his family member or close friend a haircut without being paid, but merely as a favor, would this cause the degradation the RamBam speaks of? It seems as though the Cesef Mishneh states that it has to be done as a "profession", is there anyone else that comments on this?
ב"ה Shalom It is always refreshing to get a question which makes our daily tefilot of איק return םכ the Cohanim to the Bet Hamikdash to renew the Avodah as relevant to our times. It is known that the Chafetz Chaim zt"l, who himself was a Cohen, felt that when Jews started returning to Eretz Yisrael last century that the G'eula is imminent and started urging Cohanim to study Seder Kodshim. He also turned to Rav Kook zt"l in that matter, and when Rav Kook zt"l resided in Yerushalayim he established a Kolel to study Kodshim and the laws of the Korbanot. In any case, I did not find anyone who disagrees with the Keseph Mishneh. What I did find is an interesting Tashbetz (תשב"ץ ח"א סי' ט"ז) . While basing himself on the Keseph Mishneh, he deduces from the Rambam, that the fact that the Rambam did not enumerate all the different professions mentioned in the Gemara in Kiddushin (דף פ"ב ע"א) which is the source of the Rambam, which means that the status of the profession is determined by its place and time. Meaning, that the Rambam did not see all the professions mentioned in the Gemara as undesirable ones, since in his location they were desirable. The main thing being according to the Tashbetz is that one chose a profession clean of thievery , of foul language, thoughts of sin and of physical filth. This opinion is supported also by the Chida (חיים שאל ח"א סי' פ"ז) and by the Sde Chemed חלק א , מערכת האלף כללים אות קס, ממש בסןף)) who found the words of the Tashbetz so important he quoted his whole Teshuva. All the best