I'm so sorry this happened to Sam. You are lucky that the owner paid for the vet bill (well most of it), I've seen numerous dogs come into the clinics that have been attacked by big dogs, and the majority of these owners of the big dogs are highly irresponsible and extremely uneducated about responsible dog ownership. And they all have 'no money' which is another huge issue. Your dog attacks another dog, that is ON YOU!
Were the dogs off leash with the owner nearby? Or did they get out somehow? You said they appeared out of nowhere so I'm assuming got out somehow. Do you know if they are neutered/spayed? Bully breeds, as most of us know, are animal aggressive and have a very high prey drive. And unfortunately when they are off-leash and/or get out, they tend to 'gang up' and attack. This is not true with all bully breeds, but this tends to be the cause with the aggressive ones.

I hope everything turns out okay, and I hope that Sam heals up soon! Keep up updated, too!