“And you shall make him holy” (21; 5)

The Talmud Yerushalmi says on this verse (Brachot 8; 5) 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 who rely on serving others for their livelihood (as workers, servants, or retailers). Therefore the poskim have ruled that if a cohen receives pay for his work, it is possible for him to forego his honour. The sanctity of the cohen cannot be greater than this, because the money that he earns is like a redemption for the Temple 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 would have oaid servants who were cohanim (look in Chulin 133a)

Furthermore we find that the Sages were not specific that it has to be a tangible gain for the cohen, for example paying him money, but even if he receives a spiritual gain one is able to make use of the cohen. We find in Bava Kamma 20a that Rav Chisda 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 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). It is known that Rav Chisda was a cohen (look in Shabbat 10b). So we see that even 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. 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 Avot 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 Mishna. Even so, the gemara there brings an incident with Reish Lakish who needed to ford 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 Mishna, 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’. It is explained there that Reish Lakish accepted his answer, and was no longer upset. It seems from here that even if a person is happy to be made use of, that is also considered some kind of reward, and 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 and not a regular person, it may apply also 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 honour), so perhaps we can extend it to a cohen.
Some people 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 (Bechorot 37b). It is clear from this that a cohan can be a slave.

However the truth is that 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. For the pay that he earns as a slave he pays back the theft. But someone who sells themselves does not have their ear pierced. Since this Talmud is only talking about a thief, therefore he has cheapened his cohanic status through his actions and the prohibition of making use of a cohen would no longer apply.

From everything we have explained, it is very painful to read that one of the greatest poskim (the kinesset hagedolah) gives a harsh ruling on this matter. He writes (OC hagahot Beit Yosef 31): Experience teaches us that someone who makes use of a cohen will not see any blessing in that thing.

From his words it implies that even if the cohen receives pay for his work it won’t help. This is a very difficult ruling, but its status is doubtful.