What does your lease say? Did you OK the dog through management before you got it? If your lease has restrictions there isn't a lot you can do (especially if you got the dog knowing that they don't allow certain breeds). If, however, there are no restrictions in your lease as to what type of dog you can have AND the management allows dogs of this size, you can try and fight it. Most of the time it isn't the apartment management that is the problem it is the insurance companies that they buy liability insurance from. If this is the case you can see if they will allow you to purchase insurance yourself to cover any damage the dog could do. Otherwise, I'd be looking for another place to live.
As long as they approved the specific dog you have when you signed your pet addendum and you paid your pet fees then there is really nothing they can do until the end of your current lease because they are violating the lease agreement. If that is the case, you may want to consider hiring an attorney. However if they didn't approve the dog in question or didn't know about the dog, then there is nothing you can do to fight it. Regardless though you may want to start looking for a new place to live and better yet, maybe if possible some place to buy instead of rent. This is a very hard breed to rent with. If you continue to rent, you may want to look into renting a place from a private landlord instead of an apartment complex.
It doesn't matter what they TOLD you it matters what is WRITTEN. Landlord can tell you you can have dogs 'till he's blue in the face and then deny it later and evict you for it. Always get everything in writing.