The Talmud Yerushalmi (Brachos 8; 5) says on this verse that it is forbidden to make use of a Cohen, and one who does so is considered as if they used things sanctified to the Temple.
However, since it is very difficult in practice to keep this law, since there are many Cohanim and their families rely on serving others for their livelihood (as workers, servants, or retailers). Therefore, the poskim ruled that if a Cohen receives pay for his work, it is possible for him to forego his honor. The sanctity of the Cohen cannot be made less, since the money that he earns is like a redemption for a Bedek haBayis item (certain Temple items can be redeemed for money and lose their holy status in the process). We find that even Sages of the Talmud had servants who were Cohanim (see Chulin 133a).
Furthermore, the Sages were not specific that it has to be a tangible gain for the Cohen (example paying him money), but even if one receives a spiritual gain one is able to make use of the Cohen. Rav Chisda (Bava Kama 20) asked a certain Sage for the source of a particular Halacha that he didn’t know. The Sage replied with the condition that Rav Chisda, a Cohen (see Shabbos 10b) must first serve him in some way. (Rav Chisda placed a turban on his head, to show that he was accepting him as his teacher temporarily). Thus, for a spiritual gain of learning Torah it is permitted to make use of a Cohen. Furthermore, from here we can say that even if there is no physical or spiritual benefit, but it suits the Cohen to serve, it is permitted for him for work for others.
It is well known that just as it is forbidden to make use of a Cohen it is also forbidden to make use of a Talmid Chacham. This is the meaning of the Mishna (Pirkei Avos 1; 7) ‘One who makes use of the crown (of Torah) will depart from the world’. Also in Megillah (28b) they said that it is forbidden to make use of someone who has learnt Mishnah. Even so, the Gemara there brings an incident which Reish Lakish needed to cross a river, and someone came and carried him across the river on his shoulders. Reish Lakish asked him if he had ever learnt Chumash or Mishnah, and the man replied that he had. Reish Lakish was very upset that he had made use of a Talmid Chacham. The man said to him ‘it is good for me to be of service to the master’. Reish Lakish accepted his answer, and was no longer upset. Even if a person is happy to be made use of, that is also considered some kind of reward, and yet is permitted.
Perhaps, we can extend this concept also to making use of a Cohen. Even though this reasoning is only said about a Talmid Chacham, it may also apply to a Cohen. The Talmud (Kiddushin 33b) says that someone who is involved in good deeds is also considered like a Talmid Chacham (in regards to standing up before them, and giving them honor), so perhaps we can extend it to a Cohen.
Some want to bring a proof that it is permitted to make use of a Cohen from the halacha that the slave who is a Cohen does not get his ear pierced (if he wishes to extend his servitude beyond the seven years), because it would blemish him (Bechoros 37b). It is clear from this that a Cohen can be a slave.
However, this is not a good proof, because it is known that piercing the ear is only for a slave that was sold by the Beit Din, because he was a thief who could not repay what he stole. But someone who sells themselves does not have their ear pierced. Since this Gemara is only talking about a thief, therefore such a person has cheapened his Cohanic status through his actions and the prohibition of making use of a Cohen may no longer apply.
From everything we have explained, it is very painful to read that one of the greatest poskim (the Kenesset haGedolah) gives a harsh ruling on this matter. He writes (O.C. hagahos Beit Yosef 31): “Experience teaches us that someone who makes use of a Cohen will not see any blessing in that thing.”
