What is the ruling on a pharmacist agreeing with doctors in a rented building to refer patients to him so that they can obtain medicine from him without a percentage of the rent?

If the intention is to exempt the doctor from a fee or part of it in exchange for referring patients to make purchases, then this is permissible in principle as a commission or brokerage fee, provided that it does not lead to a prohibited outcome, such as directing a patient to purchase a medicine that will not benefit him. Avoiding this is safer.