What is the ruling on the difference in price between the original cost of a travel visa and the price at which it was sold to other individuals without their knowledge, given that all procedures are legitimate and involve no deception or fraud?

The questioner did not specify the nature of his work. If he is an agent for both parties or for one of them, then it is not permissible for him to take the difference in price from the one who appointed him. However, if he is a broker (middleman), then his work is permissible, and his commission is also permissible, provided that a specific, known commission is agreed upon by both parties, the broker, and the one requesting the service. It is not a condition for the workers to know the broker's commission from the employer, nor for the employer to know his commission from the worker.