Gemara

(Beraisa - Tana d'Vei R. Yishmael): "V'Rapo Yerapei" permits doctors to heal.

Berachos 60a (Rav Acha): One who goes to let blood prays "may this be for a cure. May You heal me, for You are a faithful healer. Your cure is a true cure". It is improper for people to engage in Refu'ah, but they accustomed themselves to.

Objection (Abaye - Beraisa): The Torah permits doctors to heal!

Yevamos 106b: Rav Papa's wife's sister fell to an unworthy Yavam. Rav Papa told him to do Chalitzah on condition that she will pay him 200 Zuz. Afterwards, Abaye said that she should pay him. Rav Papa said that she was just fooling him.

(Beraisa): If a man was fleeing jail, and offered the ferryman a Dinar (a large amount) to take him across, he owes only the usual fare.

He can say that he was just joking. Also she can say that she was just joking!

Rishonim

Rosh (Nedarim 4:8): Refu'ah is a Mitzvah. The Yerushalmi says that a Choleh does not merit to be cured by everyone.

Ramban (cited in Beis Yosef YD 336 DH u'Mah she'Chosav she'Lo): The Torah permits a doctor to heal. One might have thought that it is forbidden lest he err, or lest people say 'Hashem strikes, and he heals.' It is not natural for people to engage in Refu'ah; it is merely their custom. It is Reshus, i.e. a Mitzvah to heal. Doctors may authorize people to eat Isur. We Mechalel Shabbos for Refu'ah. This shows that it is Piku'ach Nefesh, which is a great Mitzvah. It is praiseworthy to be zealous about Piku'ach Nefesh. One who asks (if one may be Mechalel) spills blood, all the more so if a qualified doctor totally refrains!

Poskim

Shulchan Aruch (YD 336:1): The Torah permits doctors to heal. It is a Mitzvah. It is part of Piku'ach Nefesh.

Taz (1): If it is a Mitzvah, why is it called Reshus? It seems that true Refu'ah is through asking mercy from Hash-m - 'I struck, and I heal." However, not all merit this, so one must engage in 'natural' cures. Hash-m consented, and gave Refu'ah through nature. This is the Torah's Reshus. Now, it is obligatory to cure. Rav Acha holds that one must apologize for seeking natural Refu'ah, like the custom of people, and acknowledge that true Refu'ah is from Hash-m. Abaye disagrees, for the Torah consented.

Yad Avraham: It is permitted only if he knows that his Refu'ah is from Hash-m, and does not depend on the medicine or the doctor.

Shulchan Aruch (ibid.): One who refrains from this spills blood, even if there is someone else to heal the Choleh, for one does not merit to be healed from everyone. However, one should engage in Refu'ah only if he is an expert and no one there is better than him. If not, he spills blood.

Gra (4): One may not rule in Halachah where there is someone greater. All the more so this applies to Refu'ah! Accidentally making one bleed is an Isur b'Shogeg (Sanhedrin 84b). The best doctor goes to Gehinom (Kidushin 82a).

Shulchan Aruch (ibid.): If one cured without Reshus (permission) of Beis Din, he must pay, even if he is an expert. If one cured with Reshus of Beis Din, and erred and damaged, he is exempt b'Yedei Adam, and liable b'Yedei Shamayim. If he killed and found out his mistake, he goes to Galus.

Ramban (in Beis Yosef DH u'Mah Aval): A Tosefta (9:3) teaches that if a doctor cured with Reshus and killed, he is exiled. This suggests that he is punished like for Shogeg. Really, he is like a judge who is commanded to judge. If he unknowingly erred, he is not liable at all; one judges according to what he sees. Still, if Beis Din learns that he erred, in certain cases he must pay. If a judge had Reshus of Beis Din he is exempt. Here he is exempt b'Yedei Adam, but liable b'Yedei Shamayim he must pay for damage or go to Galus for death. If he did not learn of his mistake, he is not liable at all.

Shulchan Aruch (2): A doctor may not take wages for his Chachmah and Limud (teaching). He may take for his toil.

Tur: He may not wages for what he learned, for the Torah obligates returning an Aveidah of the body - "va'Hashevoso Lo." The Torah obligates doing Mitzvos for free. He may take for his toil and for wages he lost

Beis Yosef (DH ul'Inyan): The Ramban concludes 'wages for his Chachmah and Limud are forbidden, like wages for sprinkling and Kidush (with ashes of Parah Adumah). Wages for his toil are permitted, like wages for bringing and filling. All the more so, he may take for wages he lost, like the Mishnah says.

Taz (2 and Shach 5): 'Limud' refers to teaching the Choleh (or others who administer it) how to administer the cure.

Darchei Moshe (1): The Nimukei Yosef says that the Ritva disagrees. He is paid only like an idle worker, or his proper wage. Binyamin Ze'ev (280) says that he must pay everything he stipulated, for it is normal to pay much to a doctor at times. Also Semag (Aseh 74, 153a) says so.

Shulchan Aruch (3): If Levi has medicine and David is sick and needs it, Levi may not charge more than its proper value. Further, even if they stipulated a high price, due to the need at the time, for David could not find anyone else with it, Levi gets only its value. However, if one hired a doctor for a high fee he must pay it, for he sold his Chachmah, and this has no price.

Beis Yosef (DH Mi): The Ramban derives this from Yevamos 106, and also the Rashba (brought in Magid Mishneh (Hilchos Gezeilah 12:7).

Bedek ha'Bayis: The Mordechai (Yevamos 24,25) says that if he paid him, he cannot take back the money.

Rema: Even though he has a Mitzvah to heal him, any Mitzvah incumbent on everyone, if it happened to come to a person and he wanted to fulfill it only for money, we do not take it from him, nor do we uproot their obligation to pay him.

Beis Yosef (DH Kosav, citing the Ramban): Some say that the ferryman gets only what he would want to be idle, for Hashavas Aveidah obligates him to save the man. If so, a doctor is paid only for the wages he lost, even if he stipulated for more. This is unreasonable. In Yevamos we learned from the ferryman to Chalitzah. The Yavam was not obliged to do Chalitzah. He could do Yibum! Rather, the Halachah is that one who stipulates under duress is obligated only the usual amount. A doctor is worth whatever he charges, so one must pay it, even though he is commanded to heal. If a Mitzvas Aseh incumbent on everyone came to a person and he wanted to fulfill it only for money, we do not take it from back him or uproot the obligation to pay him.

Taz (3): One may not take money for Limud and Chachmah, but since he stipulated and the Choleh obligated himself, he is liable, for the Mitzvah to heal is not only on him. Anyone can learn Refu'ah! The ferryman gets only his normal wage because there is a fixed fee. This does not apply to Refu'ah. A stipulation helps because the doctor was not obligated to learn Refu'ah.

Einayim l'Mishpat (Yevamos 106b:5): It is not clear whether it suffices that it is a Mitzvah for him to save, or if we also require that no one else can save.