What is the ruling on the action of a doctor who buys medicine for 500 dirhams for a patient, then gives the patient part of it and distributes the remainder to other patients without the buyer's permission?

The doctor is not permitted to prevent you from taking medicine you have purchased, even if it is in excess of your needs, unless you voluntarily donate it. This is based on the Prophet's (peace be upon him) saying: "The property of a Muslim is not lawful except with his willing consent."