What is the ruling on granting sick leave to a patient whose complaint's validity is difficult for the doctor to ascertain with certainty?

If the illness is apparent, there is no issue in granting the patient leave. However, if it is not apparent, then one should assume the best of the Muslim and grant them appropriate leave, unless evidence reveals their falsehood.

There is no sin upon one who grants leave based on the applicant's statement, even if the applicant was lying, because judgment is based on what is apparent. The Prophet, peace and blessings be upon him, said: "Indeed, I am only a human being, and you bring your disputes to me. Perhaps some of you are more eloquent in presenting their arguments than others, and I judge in their favor based on what I hear. So, whoever I rule in favor of, concerning a right of his brother, let him not take it, for I am only cutting off for him a piece of fire."

However, what is required is investigation and not assisting the applicant in taking leave they do not deserve, because a doctor granting sick leave to someone who is not ill, or more than they deserve, is considered a false statement and false testimony. The doer of this commits a great sin, and likewise, the one who takes it also sins, because they have taken money unjustly.