Once the auction is done and you won it you pay the transfer fee (the offer you made to win the auction). When your squad is not full (22 players) the player automatically signs a contract for his annual salary (the annual salary is not pais at once, but a part of it each matchday). If your squad was already full, your player is parked in a queue and must you must sell another player to sign him finally (you have one week time to do so). To activate him, there appears a dialog, if you really want to sign him for his salary. It is like in the upper case, you don't pay the salary when you sign him, but part of it each matchday. So there is no additional signing fee.