Personally, I'd say let the barn owner take it as legally that's his right due to the past due fees. He seems like a stand up man and he's been taking good care of BC for three months now without being paid, so I'd imagine he'd continue to be good with training, etc. If he later sells BC to a good home to recoup some of the money he's put in for training, etc., that's certainly his right, logical and a paying (new) owner is probably going to take better care of BC BECAUSE they'll be paying good money to get a well trained and very wanted BC. (As opposed to the girl who paid $1 - nothing big invested, nothing big to walk away from, you know? I KNOW you were being super nice, please don't feel I'm picking on you about that, it was very sweet of you.)
If you decide you want BC back and want to take on the training and expenses, the first expense you have to take care of, IMO, would be paying the past due stable fees. If the BO is willing to let the girl give BC back to you, it's not fair that he lose the money for the three months he's been taking care of BC.