What is the ruling on receiving a sum of money in exchange for referring a customer to a bank for car financing? Is it permissible to refer the customer to a non-Islamic bank if the Islamic bank refuses? Is it permissible to sell to a non-Islamic bank so that it, in turn, sells to people?

It is permissible to take 500 dollars for directing a customer to a bank if the bank buys the car from you and then sells it to the customer. This combines brokerage and sale. However, if the bank's role is financing and paying on behalf of the customer without buying the car from you, this is an usurious loan and is forbidden, and it is not permissible to take a fee for directing someone to it. It is permissible to sell the car to a usurious bank that then sells it to customers, because lawful dealings with an usurer are permissible.