Is the money an employee takes from a driver in exchange for bringing him a customer, or for lowering the price for the customer, lawful or unlawful?

This money is from permissible brokerage in permissible transactions. The buyer's knowledge is not a condition, as long as it is taken from the seller alone; because its condition is the knowledge of the payer alone, and the payer here is the driver, not the buyer. Therefore, there is no harm in taking it from him, even if the buyer does not know about it.