[A Taxi Driver Handing Over His Passengers to Another for a Fee]

Answer: What appears to me is that it is permissible. The first driver is lawfully entitled to the amount paid to him in exchange for handing the passengers over to the other car; in reality, that amount is a consideration for his having gathered the passengers.
If it is said: Gathering the passengers may only be through great effort, yet sometimes they assemble for him without any effort—Is the ruling the same?
We say: The norm for one who gathers passengers is that it requires toil and added expense; at times they may assemble for the car without any expense. Monetary compensation is permissible for what ordinarily requires cost and effort—whether in a given instance it required cost or not.
If it is said: Why do you not say that what is mentioned in the question is a sale of rights, and the sale of rights is invalid?
We say: We have said what preceded because it is necessary to construe Muslims’ contracts and transactions as valid whenever possible. If a transaction admits an aspect of validity and an aspect of invalidity, it is carried upon the valid aspect. Thus they have laid down in their legal maxims.
Among their maxims in this area: the default in transactions is validity.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2