What is the ruling on granting sick leave or a medical excuse to a healthy person who does not have symptoms that qualify him for it, and is anything required of the doctor or official who wishes to repent from this action?

It is not permissible for the doctor to do that, because it is explicit forgery and deception. "Whoever cheats us is not of us." He must repent sincerely, seek forgiveness, perform righteous deeds, and resolve never to return to such an act. If his action results in the loss of a right, he is liable for that, because causation is one of the causes of liability. The doctor is liable for what the employee's employer pays out that is not deserved because of the doctor's certificate, and he can demand reimbursement from the employee for what he has paid.