What is to be done about someone who had their car washed but could not find the person who washed it to pay them their wages?

You must search for the rightful owner to give him his due. If you do not find him or it is difficult for you to reach him, then give the money in charity on his behalf. If you find him later and he approves of the charity, then you are absolved. Otherwise, you must pay him the money, and you will receive the reward for the charity.