I agree, you should go over there and just take her. What are they going to do, call the police? I don't think they really want to go there. Take a friend with you and show up unannounced, but at a time you know they will be home (spy if necessary!). Try to get Butters on your lap or next to you.
First, try to negotiate with them. Since they didn't pay for her, I don't think you should pay them back for the dog. But you could offer to pay them for babysitting her and for food and stuff. See if you could work out an agreeable amount and payment schedule.
If negotiation doesn't work, I would pull out any paperwork I had saying that she was yours, including vet records, breeder records, licensing, etc. I would then just say that you tried to work it out nicely, but since Butters was rehomed without your permission, you are going to take her back now. Hopefully Butters is on your lap or next to you at this point. Stand up and you and your friend walk out of there.
Free Butters!
