Question: Why can and should a doctor accept payment for treating on Shabbos but a chazzan cannot be paid for davening on Shabbos, it must be part of an annual or monthly salary?
A. The Mishna Berura (306:5:24) Say it is forbidden to hire a chazzan to pray on Shabbos, but some allow it, but if his salary is for the year or for the month everyone agrees it is permissible. Those that said you can’t are because you can’t take wages on Shabbos. Those that say you can are because for a mitzvah you can, the rabbis didn’t make any decrees against it but you won’t see any blessing coming from the money earned on Shabbos… But a doula definitely is permitted to take wages for Shabbos.
B. In Dirshu footnote 21 the Steipler says that even if a doctor does not want to take money for his work on Shabbos it should still be given to him, at least as a gift.
C. The Pri Megadim says the reason one can take wages for healing is because he is being paid for saving one’s life. But it is still for a mitzvah so why is it different than a Chazzan davening. They are both permitted to do what they do on Shabbos, but one should take payment for what he did on Shabbos and the other shouldn’t technically?
Answer: Healing isn’t just permitted on Shabbos to save a life, it pushes off Shabbos, so once Shabbos is pushed off then it’s permitted to charge, especially since if he does not accept payment this time he might not heal or be lazy to do his job next time on Shabbos and for life and death we’ll do pretty much anything to save a potential life in danger. But a chazzan, though what he is doing is a mitzva by leading the minyan in davening but he is not pushing off Shabbos, so the rabbinic prohibition of accepting wages for Shabbos is still prohibited according to this opinion and certainly will not see any blessing.