Is what the beggar took from the employer's money without his knowledge considered unlawful (haram), and how can it be returned to its owner if he cannot be reached?

If the contract between you and the company owner stipulates that you are entitled to five riyals for each trip and he has not paid it to you, then it is permissible for you to take it without his knowledge. However, if the contract does not stipulate this, then it is not permissible for you to take it, and the money must be returned to its owner or his heirs. If you do not find them, then give the amount in charity on behalf of its owner. If you find them later and they approve of your charity, then the reward is theirs. Otherwise, you must pay them the money, and the reward for the charity is yours.