AFAIK, there's nothing you can do to save the fox if it has mange, short of live-trappiing it and having a vet treat it. In our area the fox die from it (hypothermia, actually) and the coyotes somehow survive it. Wiped out a whole litter AND the vixen in my backyard a couple of years ago - and she had a nice burrow.

I don't recall this being a big problem here pre-coyote, FWIW.