I am also appalled after reading through this thread. Unethical, and heartless, at the very least.
One question: would they really have let Pax die right then and there? Just because you didn't have all the funds available immediately? Even human ERs take in people- all people, no matter what their circumstances are, and do what they can to save them because they have all taken an oath (Hippocratic): "First, Do No Harm." Afterwards is when the financial stuff gets sorted out...
I would pursue this, for sure. No matter how uncomfortable and difficult it may be for you to do so.
Also, it may be a nice, happy thought to believe that Pax is going into the hands of some good people who will care for him and love him as much as you did...but without any contact whatsoever, how can you know that for sure? If it were me, I would not be able to sleep at night....worrying about my boy.
And I do not mean this as a guilt trip at all---believe me, I am not made of money, and wonder sometimes how I would handle things if Mateo needed a really huge medical treatment...
I just really hope that you will follow up with him, the hospital and this mysterious vet...through legal and/or whatever means possible.