Can a veterinary establishment keep your animal if you can't pay your bill, and...can they add charges after you leave?

I had a veterinarians establishment provide care to my pet pygmy goat, requiring him to stay 'in-patient' for a week. When my little goat was ready to go home and I went to the office to "check him out", they presented me with his $1,200 bill and demanded at least half of it to be paid or else they would not release my goat to me! I borrowed money to pay and be able to get my goat back. Then, later, when my bill arrived...they had added another $600 in expenses to it, AFTER they had made me pay half of what they said was the final bill upon checking him out. Is there anything I can do about this, and was what they did even legal? I am still horrified at the meanness of these folks. Sandy