In practice their commission is determined by player’s salary that they negotiate on behalf of the player. The bigger the salary, the bigger the commission. Not only do they absolutely work for the players, they have a fiduciary duty to them.
the original sin here is the whole structure of agent compensation in football — in practice agents do not work for players because their pay is primarily determined by transfer fees paid for the player and the cut of that fee they can finagle it's exploitive from the ground up