All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.
It is not permissible to check patients using this apparatus while they are not aware of its condition because they visit the dispensary and pay their money to be checked carefully using a good and serviceable machine. The machine is supposed to reveal the disease from which the patient suffers. As you have said, the apparatus does not perform this task as required and, therefore, it will be deceptive to use it. Deception is forbidden, as the Prophet, sallallaahu ‘alayhi wa sallam, was reported to have said: “Whoever deceives us is not one of us.” [Muslim]
Deception is not an Islamic trait, and Muslims should avoid it in their dealings. People who work in this dispensary should explain to the owner the danger of what he is doing. In addition to the deception involved, the lives of the people are put in danger and their money is consumed unrightfully. Allaah The Almighty Says (what means): {O you who have believed, do not consume one another's wealth unjustly.} [Quran 4:29]
The money earned from using this apparatus in its current condition is not permissible for the owner to possess or utilize. He must return the money to the people who paid it or to their inheritors. If he does not find them, he should give it in charity on their behalf, after exhausting the means to seek them.
Allaah Knows best.