that really sucks. So it seems you were a foster family for this group?
Its definitely a little shady that they would come and get the dog while he was technically still in your care while at the vet. I would do as the others say and try to get someone to adopt him for you. Judging by how your relationship has deteriorated with the group, I wouldnt think it would be near impossible for you to adopt the dog on your own.
I had experience with a group similar to this, Pets without Parents, thankfully they seem to be a little more established and were very professional and I had a great experience. I know it doesnt help this situation, but for others looking to foster or adopt, Pets without Parents was very good in both regards.