What is the ruling on taking extra money in exchange for facilitating or following up on a transaction, and is that considered brokerage, and what is the ruling on brokerage?

If an employee receives a salary from the state for his work, then it is forbidden for him to take additional money, and this is considered a prohibited bribe. However, if he takes money for work that he is not obligated to perform, then it is permissible to take it as a ju'alah (commission/fee).