Is it permissible for a doctor to receive a monthly salary along with a percentage of the clinic's profits, and does the ruling differ among the various schools of Islamic jurisprudence?

Leasing is a compensatory contract for the ownership of benefits, and it is legislated in the Qur'an, Sunnah, and by consensus, as in His (Allah's) saying, "We apportion among them their livelihood in the life of this world, and We raise some of them above others in ranks, so that some of them may take others for service" and "One of them said, 'O my father, hire him. Indeed, the best of those you could hire is the strong and trustworthy'" and "And if they suckle for you, give them their due payment." Al-Bukhari narrated from Abu Hurairah that the Prophet, peace and blessings be upon him, said: "Allah the Almighty said: 'Three types of people I shall be their opponent on the Day of Resurrection: a man who gives a promise in My Name and then breaks it; a man who sells a free person (as a slave) and eats his price; and a man who hires a laborer and gets full work from him but does not pay him his wages.'"

Accordingly, it is permissible to enter into a lease contract with a known salary and anything in excess of it as an incentive bonus. There is no objection to this, as long as the lease has fulfilled its conditions of mutual consent of the contracting parties and knowledge of the benefit.