Is it obligatory to pay the hospital the cost of the operation, even if the patient was not informed of it beforehand, and considers it fraud and deception due to its exorbitance?

For the validity of a rental contract, the rent must be known. If it is unknown in a way that leads to dispute, the contract becomes void, and a fair wage ( اجر المثل ) becomes obligatory. An exception to this is when the rent is known by custom, such as riding public transportation or giving a garment to a laundryman or tailor. In such cases, the customary wage is paid, because prevailing custom takes the place of a verbal agreement. If the demanded rent is within the limits of the customary wage, it must be paid. Otherwise, a fair wage ( اجر المثل ) determined by experts becomes obligatory.