What is the ruling on a taxi driver taking a cash commission from passenger transport offices in exchange for bringing them customers, and what should he do with the money earned if the commission is unlawful?

The questioner's situation is one of two: First, if he is an employee, it is not permissible for him to undertake additional work without his employer's permission. If his employer grants permission and the additional work does not involve a Shariah-prohibited matter, then the earning is lawful; otherwise, it is unlawful. Second, if he is not an employee and directs customers to offices in a manner that does not involve a Shariah-prohibited matter, and the commission is specified, then there is no harm in him taking the commission, as it falls under the permissible category of brokerage. If the earning is unlawful, the way to dispose of it is to spend it in good causes with the intention of disposal, not charity.