What is the ruling on a doctor replacing a patient's medication, which the patient purchased, with another medication that the doctor possesses, in order to benefit from the patient's medication having a longer expiry date?

It is not permissible to substitute one medicine for another except with the owner's permission and consent, because substitution is a type of sale (barter), and its condition is the satisfaction of both parties, based on the words of Allah Almighty: "O you who have believed, do not consume one another's property unjustly but only [in lawful] business by mutual consent," and the words of the Prophet, peace and blessings be upon him: "Indeed, selling is by mutual consent."