Gemara

21b (Beraisa - Chachamim): A Kohen cannot become a Nirtza, for this would blemish him.

(Rabah bar Rav Shilo): We learn from "he will return to his family" - to the status quo of his family. (Kohanim serve in the Mikdash.)

32a (R. Yitzchak bar Shilo): A father can pardon the honor due to him. A Rebbi cannot;

(Rav Yosef): Even a Rebbi can pardon his honor - "Hash-m went in front of them (Yisrael) by day (in a cloud)."

Question (Rava): That is not comparable! Hash-m owns the whole world, so He can pardon His honor. A Rebbi does not own his Torah!

Retraction (Rava): A Rebbi owns his Torah - "in his Torah he will think day and night"!

(Rav Ashi): Even according to the opinion that a Nasi can pardon his honor, a king cannot. "You will put a king upon you" - his fear should be upon you.

Gitin 59b (Beraisa): The one who blessed on the bread is the first to take from the bowl (to accompany his bread). He may honor his Rebbi or one greater than himself by letting them take first.

(Rabah): This applies only to a meal. Regarding reading the Torah, a Kohen may not honor another by letting him read first, lest this lead to quarrels.

Yevamos 88b (Beraisa): "V'Kidashto" - b'Al Korcho (against his will). If he married a woman forbidden to him and did not want to leave, Dafno (force him).

Rishonim

Sefer ha'Mitzvos (Aseh 32): We are commanded to honor the seed of Aharon, to elevate them and put them first and foremost. The Sifra (1:14) says "v'Kidashto" - b'Al Korcho. I.e. we are commanded. It is not his choice.

Rosh (Gitin 5:20): We let a Kohen take a nice portion first due to v'Kidashto. A Kohen should not seek to take the nice portion. One who does so is not blessed.

Mordechai (Gitin 401): A Chacham's honor is due to his Torah, and he can pardon it. All the more so a Kohen can pardon his honor, which was given to him and his seed forever - "Kehunaschem (it belongs to them). It is unlike kingship, which is Hash-m's. "Som Tasim Alecha Melech" teaches that a king cannot pardon his honor. A Kohen may pardon honor or profit unless it will lead to fights, e.g. Kri'as ha'Torah on Shabbos. "V'Kidashto" is b'Al Korcho if he marries a forbidden woman, but not to matters of honor. Surely he need not take a nice portion first! If he hates gifts, he will be blessed! He may pardon his honor at a meal, even if many are there. The Ba'al ha'Bayis may tell anyone he wants to lead Birkas ha'Mazon. R. Simchah allows a Kohen to let a Yisrael lead only if it is his Rebbi or someone greater than himself.

Hagahah: If so, if the Ba'al ha'Bayis asked a guest (a Zar) to lead, the Zar need not ask 'Reshus' from the Kohanim!

Mordechai (Gitin 461): Once, a Kohen poured water on R. Tam. The Yerushalmi says that one who used a Kohen was Mo'el! R. Tam replied that their Kedushah is only when their Bigdei Kehunah are on them (Zevachim 17b). He was asked why we must honor them at all nowadays, and was silent. R. Peter answered that they have Kedushah, and can pardon it. A Kohen cannot become a Nirtza, for it is a Mum. His master can work with him when he pardoned his honor.

Note: It would seem that this applies only to a Kohen who sold himself. If a Kohen did not want to be sold for his theft, it seems that he does not pardon!

Question (Shiltei ha'Giborim under Mordechai, Berachos 7:3): The Ba'al ha'Bayis blesses ha'Motzi, even if there is a Kohen. How can the Kohen pardon his honor? He cannot allow a Yisrael to read the Torah first!

Answer (Yad Aharon): A Kohen may honor someone else at a meal (Gitin 59b). Also, Maharam says that a Kohen can pardon his honor.

Rebuttal (Shirei Berachah (on Birkei Yosef) 4): What is R. Peter's proof that pardon helps? A Kohen sold for theft does not act like your nation! His proof is that if not for the Mum, a Kohen could be Nirtza and serve longer, even after he finished his required Avodah. Many disagree. Hagahos Maimoniyos (Hilchos Avadim 3:2) could not answer why a Kohen may be an Eved Ivri. He did not say that pardon helps, like his Rebbi (Maharam). The Semag says that it is because he is paid. Shiltei ha'Giborim holds like R. Simchah, who allows a Kohen to put before himself only his Rebbi or someone greater than himself, like the Gemara says. Why may he allow the Ba'al ha'Bayis to bless?

Poskim

Rema (OC 128:45): One may not use a Kohen even nowadays, for this is like Me'ilah with Hekdesh, unless he pardoned his honor.

Question (Taz 39): We are Mekadesh a Kohen b'Al Korcho (Yevamos 88b)! He cannot pardon his Kedushah!

Answer (Shirei Berachah 4): A Kohen can pardon honor, but not disgrace. The Tur forbids a Chacham to return an Aveidah if it entails disgrace, but permits a Kohen Chacham to allow a Yisrael to bless first. The Rema forbids Hashavas Aveidah if it disgraces the Chacham, but permits a Kohen to serve someone. This is difficult. We should be more lenient about a Mitzvah (Hashavas Aveidah)! Also, perhaps the Torah merely exempts the Chacham from returning! The Rambam permits Hashavas Aveidah even if it disgraces the Chacham, yet Sefer ha'Mitzvos forbids a Kohen to serve someone!

Taz (39): He can pardon because he enjoys this. The Torah did not forbid what is good for him. The Torah says 'in order that your ox will rest (on Shabbos)', but one may let his ox detach grass. The ox will not be serene if it cannot! Similarly, the Kohen who served R. Tam was happy that he merited to serve Kodesh. This is his Kedushah! R. Tam was not silent because he could not answer. Rather, he did not want to say 'it is permitted because I am a Chacham.' If the Kohen does not benefit from serving others, it is forbidden and like Me'ilah, even if he pardons. The Levushsee learns from "v'Kidashto" that we are commanded to be Mekadesh him, but he need not consider himself Kadosh. This is wrong. V'Kidashto is b'Al Korcho! Rather, it is like I wrote.

Rebuttal (Chachmas Shlomo): Tosfos (Sotah 41b DH Mitzvah) forbids a king to stand for a Chacham, even if he wants! The honor of Kohanim is more due to Hash-m than that of kings - "Ki Es Lechem Elokecha Hu Makriv"! All the more so they may not pardon their honor! Chachamim are called Kohanim ("u'Vnei David Kohanim - Nedarim 62a), and they have precedence over Kohanim, so surely they may use Kohanim. According to the Taz, why is service unlike letting a Yisrael bless first, for which pardon helps?

Shirei Berachah (4): R. Tam was the Gadol ha'Dor. He was like everyone's Rebbi. Perhaps the Kohen was obligated to serve him. R. Peter allows a Kohen to pardon, even if he does not benefit at all. One should not rely on this to use a Kohen, even an Am ha'Aretz, for many disagree, unless the Kohen serves a Chacham or is paid.

Minchas Chinuch (269 DH Ach): We hold that we do not force a Kohen to divorce a forbidden wife more than we force a Yisrael. V'Kidashto is never b'Al Korcho, for he can pardon honor, like the Mordechai says.

Meshech Chachmah (Emor DH v'Kidashto): Tana'im (Tosefta Sanhedrin 4:1) argue about whether or not a Kohen Gadol may bathe with others. R. Eliezer forbids, due to v'Kidashto. He cannot pardon this. However, the Rambam allows a Kohen Gadol to bathe with others! He explains that all agree that a Kohen Hedyot may not pardon his Kedushah above Yisre'elim. The argument is whether or not a Kohen Gadol may pardon his Kedushah above Hedyotim. Kehunah was Batel when R. Tam died (R. Chaim Kohen would have become Tamei for him - Tosfos Kesuvos 103b), all the more so when he was alive!

Ha'Makneh (Kidushin 21b DH v'Hinei): Since a Kohen may work for pay, he may work and pardon the wages.

Magen Avraham (75): Kehunah belongs to Kohanim, so they can pardon it. Rami bar Chama asked Rav Chisda, a Kohen, to serve him.

Gra (85): We learn from Gitin that when fights will not result, a Kohen may pardon his honor.

Mishnah Berurah (175): Even though he can pardon honor, he cannot pardon usage, for it is disgraceful. L'Chatchilah, one should be stringent. Certainly one may not use him for disgraceful work.