Finally heard from the vet and it's good news! It's a fatty tumor. Vet says to expect them to happen from now on.
Max did not enjoy his surgery one bit. It's a good thing he's always been healthy because he is a horrible patient. They sent him home with a cone around his collar so he wouldn't pick at his incision. After we got home, he worked for 20 minutes to get it off. He squatted on the floor then pushed and pulled it with his front paws until it came off. Fortunately, his stitches are the dissolving type.
Maybe they were just being nice, but the people in the vet's office did tell us they think he's a very bright dog. We think he is too, which is why we call him Max the Mighty Wonder Dog.