First- I am so sorry that poor Justus (and you!) just went through this!
Second-- Make sure to report it!!! Having a record is very very important. Not just for this. I don't know what the relationship is that you have with your neighbor but it's also good to make a report of vet expenses.
Check your own hand- if you reached in to grab a collar, it's possible the blood was your own... making it even more important to determine the dog's vaccination status. If you got bit, you will probably want to at least consider a tetanus shot.
Thankfully it sounds like it could have been much, much worse. I am sad that poor Justus has to go through this, but it could have been really heartbreaking if the dog had grabbed Flyer- I'm glad you were able to grab the wee one.