What should one do who has not paid a tailor for a dressmaking "fitting" for ten years, does not know if the tailor is Muslim or not, and can no longer find him?

If there is an agreement or custom that requires payment for a "fitting" to the tailor, then the fee must be paid to him regardless of your satisfaction with his work. However, if there is no agreement or custom, you are not obligated to pay anything. In the first case, if you are unable to reach him, give the money in charity on his behalf. This ruling applies to the property of both a Muslim and a non-belligerent disbeliever, for the property of both is inviolable.