What is the ruling on paying a sum of money to a middleman in exchange for him securing a job, and what is the ruling on earnings acquired through him?

What this person is doing is called brokerage (simsarah), and it is permissible as long as it does not involve assisting in the sale of something prohibited. Brokerage is considered a form of ju’l (payment for a service). It is stipulated that the payment must be a known amount, not a percentage of the profits, due to the uncertainty and gharar (excessive risk) involved. Knowledge of the work itself is not a prerequisite. Therefore, there is no harm for this broker in taking what is given to him in return for his mediation.