There's another thread here somewhere about this bug (I'm sure it's a bug). The amount of money you actually have available to sign free agents is never as much as it says in the front office screen. In my experience it has consistently been exactly 10,000,000 dollars too low. If you scroll down in the free agent signing window it will tell you how much money it thinks you have available. Here's what mine says right now:
(During the offseason it's a bit clearer - my free agent money in the front office screen will be exactly my budget minus my payroll, while in the free agent screen it will be exactly 10M less than that)