Is it permissible to take a portion of a worker's wage in exchange for mediating between him and the employer?

It is permissible to take a fee for mediation between an employer and an employee, provided that the party from whom the mediator takes the fee is aware of it. It is also stipulated that the fee must be known at the time of the contract. So, if the worker's wage is known and a specific percentage of that wage is agreed upon, there is no objection. However, if the worker's wage is unknown at the time of the contract, and the agreement is on a percentage of their wage, then it is not valid according to the majority of scholars.