Gemara

(Beraisa - Chachamim): A Kohen cannot have his ear pierced (to remain an Eved Ivri), 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.)

Bava Kama 20b: Rav Chisda asked a question to Rami bar Chama. Rami asked Rav Chisda to serve him before answering. Rav Chisda folded Rami's turban.

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

Rambam (Hilchos Avadim 3:8): An Eved Ivri Kohen cannot become a Nirtza.

Hagahos Maimoniyos (2): This proves that one may use a Kohen. Rami bar Chama asked Rav Chisda, a Kohen, to serve him (before answering his question - Bava Kama 20b). However, the Yerushalmi says that one who uses a Kohen was Mo'el. This requires investigation.

Mishneh l'Melech (Hilchos Kinyan 3:8): Semag says that the only Isur is to use a Kohen for free. We learn from the Rivash that one may use an Am ha'Aretz Kohen. Perhaps such a Kohen may be sold. We must use the Semag's distinction to explain why Rami bar Chama used Rav Chisda, i.e. he paid him.

Nish'al David (OC 4 DH Ach, DH v'Re'ayah...): Even if Rav Chisda were not a Kohen, why was Rami bar Chama able to use his father-in-law? And even if this was before Rami married his daughter, how was he able to use a greater Chacham? Rav Chisda holds that a Chacham may not pardon his honor! We must say that Rav Chisda made himself like a commoner for the sake of Torah.

Dameshek Eliezer (128:85): V'Kidashto does not apply when both of them are Kohanim. Rav Chisda served Rami bar Chama before learning from him, for Kavod Torah is greater than Kavod Kehunah - "Yekarah Hi mi'Pninim".

Rebuttal (Kaf ha'Chayim OC 128:283 citing va'Ydaber Moshe): The Mordechai forbids a Shali'ach Tzibur Kohen to call another Kohen to read the Torah, for this disgraces his own Kehunah. Even though, this is not the Halachah, this shows that one Kohen may not serve another.

Sefer ha'Mitzvos (Aseh 32): We are commanded to honor the seed of Aharon, to elevate them and put them first and foremost. Even if they refuse, we do not heed them. This aggrandizes Hash-m, who chose them for His Avodah - "v'Kidashto Ki Es Lechem Elokecha Hu Makriv Kodesh Yihyeh Lach." The Sifra (1:14) says "v'Kidashto" - b'Al Korcho. I.e. we are commanded. It is not his choice.

Mordechai (Gitin 461): A case occurred in which a Kohen poured water on R. Tam. A Talmid asked that the Yerushalmi says that one who used a Kohen was Mo'el! R. Tam answered that they do not have Kedushah nowadays. Their Kedushah is only when their Bigdei Kehunah are on them. The Talmid asked, if so, we should not honor them at all nowadays! R. Tam was silent.

Rivash (94): We do not give Matanos to a Kohen Am ha'Aretz (Chulin 130b). We need not punish greatly one who disgraces a Kohen Am ha'Aretz, since the Kohen does not learn Torah like a Kohen should. Chachamim did not refrain from cursing Kohanim of the greatest lineage who acted improperly (Menachos 53b, Yoma 71b). All the more so this applies nowadays, when Kohanim cannot prove their lineage.

Shirei Berachah (4): The Rif, Rambam and Rosh did not bring the law that we do not give Matanos to a Kohen Am ha'Aretz. They hold that it is only a preference, like Perush ha'Mishnayos (Sof Chalah). I hold that the Isur to use a Kohen applies even to an Am ha'Aretz. However, a Talmid, even a Kohen, should serve his Rebbi. This explain why Rav Chisda served Rami bar Chama before learning a Halachah from him.

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.

Taz (39): We are Mekadesh a Kohen b'Al Korcho (Yevamos 88b)! He cannot pardon his Kedushah! He can pardon only if he enjoys this. The Kohen who poured water on 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.' It seems that if the Kohen does not benefit from serving others, it is forbidden and like Me'ilah, even if he pardons. The Levush 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.

Be'er Heitev (83): The Mordechai says that v'Kidashto is b'Al Korcho only regarding marrying forbidden women. A Kohen can pardon his honor even if he does not benefit from this.

Shirei Berachah (4): A Kohen can pardon only honor, but not disgrace. We find that a Chacham may not return an Aveidah if it will be a disgrace.

Magen Avraham (75): Kehunah belongs to Kohanim, so they can pardon it.

Machatzis ha'Shekel: If a Kohen does not pardon his honor, he may not serve even another Kohen.

Bi'ur Halachah (DH Asur): Some permit a Kohen to serve a Kohen. Some are lenient to let a Kohen Am ha'Aretz serve, but not disgraceful service.

Kaf ha'Chayim (282): All agree that if the Kohen himself decides that he wants to serve a Yisrael, one may accept. This is the Kohen's honor!

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

Nish'al David (OC 4 DH Ach, DH v'Re'ayah...): We asked (Nedarim 35b) whether Kohanim are Sheluchim of Yisrael, or of Hash-m. If they are Yisrael's, surely Kohanim may serve Yisre'elim! However, the Gemara said only that this affects a Mudar Hana'ah. In any case, we rule that they are Hash-m's Sheluchim. Even those who disagree with R. Tam permit a Kohen to serve a Kohen. Some forbid lighting a Ner from a Ner of a Mitzvah, but all permit lighting a Ner Mitzvah from another. Similarly, it is no disgrace for one Kohen to serve another, unless it is harsh labor. Rabah used Abaye to send Shaloch Manos, for also Rabah was a Kohen. The Rema forbids, so we cannot permit l'Chatchilah, but it seems that the Bavli does not forbid using a Kohen, unlike the Yerushalmi.

Mishnah Berurah (175): Some permit to use a Kohen only if the Kohen benefits, e.g. wages or he desires to serve an esteemed person. 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.

Kaf ha'Chayim (280): Many Amei ha'Aretz are careful not to marry a Bas Kohen for this reason. Pesachim 49b says that the end will be bad. Some say that even a Chacham should be careful, lest Aharon not desire that he cling to Aharon's seed. If she is an Eshes Chayil and her brothers are Chachamim and one cannot find another like her, he may pray (that it go well) and marry her l'Shem Shamayim. He must be very careful about the honor due to her.