Is the money remaining from the cost of a scope that was not performed considered illicit, and must it be given as charity if it cannot be returned, and what sin or expiation results from that?

You must pay the full endoscopy fee to the clinic, and it is not permissible to refuse payment under the pretext of injustice or lack of need; because determining the patient's need is the physician's responsibility, and you could have refused to undergo the endoscopy. Likewise, it is not sufficient to give the endoscopy fee as charity; for giving haram money as charity is only permissible if it is impossible to return the money to its owner. However, if the owner is present and it is possible to return it, then it must be returned, based on the Prophet's (PBUH) saying: "The hand is responsible for what it has taken until it returns it." You can tell them that you found the amount among the papers.