I would use you judgement on this one! You are there seeing what is going on, not the rescue. So if you feel after a few days, that Maggies health is not improving or is further going down hill, i would contact the rescue and return the dane. I hate to say this, but since how Maggie is responding to this new dog, the stress could very well kill her if things dont improve. You are right when you say you have to think about Maggie first!