What is the ruling on money received by a doctor from a private hospital in exchange for referring foreign patients to it, knowing that the insurance company pays the costs, not the patient?

It is permissible to take the amount if it is a gift, or if it was agreed upon in advance. If the hospital is the best option, then there is no harm. However, if there is a better option, then it is not permissible due to the deception involved. The fee must be known and fixed, not a variable percentage.