My dog is calm when he's WITH people, but roaring when he sees people on a distance?

My story is a bit similiar. I have a male boxer and he is a very good dog when off leash. And I never put him on it. Then one day a lady with very very very long nails moved in my tenement. She went up to pet my dog and her nails scratched my dog and made him bleed. Now, he has a problem with people petting him. What i did was take him to a trauma dog therapy unit. They really helped my dog. Look to see if you have any in your area.