I have a similar problem right now with a dog I found. He is not going through our Humane Society foster program because they are being butt heads. He is a corgi mix, and is a great little dog, but bites if you go to put on collar or leash on him. I was going to pay to have him neutered and find him a home on my own. I found a great foster home for him who is very dog savvy and understands the biting problem and is trying to work with him, but now I'm getting nervous about getting in trouble if I adopt him out, even telling someone about his issue, then having them or - god forbid - a child get bit because of it.
Mostly, he just "mouths", but he has broken the skin a couple of times. I suggested she try showing him the leash/collar, give him a treat...move teh collar/leash closer, give him a treat, etc.
right now, he has a harness on with the leash (that was a chore and a half getting those on) and we are leaving it on for him to get used to it and having it on during his comfort times - petting, sleeping, eating, etc.