How can I get the custody of my dog's puppies from a dog rescue group?

I was asked by the dog rescue group I sometimes foster for if we want to adopt the dog and that was fine but now the dog is pregnant by one of my other dogs and they want to take the dog away from me, even though they are my dog's puppies. They want to adopt them through the rescue and keep the money. My question is, since they never paid me back for the dog, who legally owns the dog at this point? I have the receipt that shows I paid for the dog. The rescue has paid some of the vet bills toward this particular dog but I am willing to reimburse them for the money they paid.