What is the ruling on the percentage a doctor receives from his work's income, in addition to his fixed salary?

If the mentioned percentage is part of the wage, then it invalidates the contract due to the unknown nature of the wage. The wage must be known, as the Prophet (peace be upon him) forbade hiring a worker until his wage is made clear to him. This is the view of the majority of scholars. Some scholars, however, went on to permit it if the wage was unknown, such as giving a garment to a tailor to sell it for half the profit. However, this contradicts the views of Malik, Abu Hanifa, and Al-Shafi'i. But if the percentage is not part of the wage, but rather a bonus or an incentive, then there is no harm in taking it.