If there is any problem with this dead tissue (necrotizing) and either you or your vet is concerned about it, it might be worth considering Maggot Therapy (bio-debridement)
If there are any feelings of horror or disgust about this treatment, I'll just mention that there have been remarkable results with this form of therapy, not only in stripping away necrotizing tissue but stimulating new growth of healthy tissue. This is used in veterinary medicine as well as Human medicine.
You might be interested to take a look at:
http://en.wikipedia.org/wiki/Maggot_therapy#Veterinary_use
Foxes -if they get into a tangle with dogs -can inflict some dreadful wounds. I'm so sorry.
I don't have personal experience of this type of wound. All I know is dead tissue will eventually fall away. The important thing is to keep the area clean, and maybe some locally-applied antibiotic cream will help, to prevent any recurring infection there. But you're probably doing all that.
Nature does usually repair with new healthy tissue in time so long as the threat of infection, or existing infection is kep under control...but like I say, I don't have personal experience of this specific thing.
I will do a little research for you, see what I can come up with if anything, and post back soon as I can.