Potential Foster Turns Reactive, Almost Got Bit

Went to see another potential foster this evening.  Was expecting a 90lb+ dog and the pics even looked it. The dog had lost a lot of weight while in shelter they claimed from depression or whatever. He was a little taller than Ember, and I could imagine if he did put on 20lbs he might be okay for the work.  He was perfect with other dogs, cats, kids, living with them all at the foster house.  He seemed fine, just lacking in some leash manners but not horrible. So I said I would give him a shot. Stopped by their local Petco for supplies. While fitting a step in harness to this dog, he became reactive.  I think it was the way I was touching him or the place around the hind legs and groin area trying to repositioning his body and legs so I could get a better angle to slip on the harness and adjust it.  He was stressing some while in the store, but still took treats.  He had tensed up and I heard a low growl, so I stopped and watched him.  There were people across from us making a bit of noise and they had a dog trying to get closer, so I thought it may have been the other dog I heard.  I started to reach back for the hind end to loop the harness through and he tensed again and turned his head slightly so I stopped again until he relaxed and was slack jaw.  Instead of leaning in to loop the harness as I had originally planned to ( which would have put my head/face right in line if he swung around on me ), I angled one arm low and came around with the other to feel at his leg and hip, wondering if he was in pain. I moved his leg slightly with my hand and there was a throaty snarl and he was turning to latch for my arm. I managed to block him with the arm I had angled against his chest for safety and held the collar so he couldn't jerk away and try again. When he realized he was partly restrained he outburst with a lot of snarling and a warning bark that had some people coming over to see if everything was okay.  Which was pretty embarrassing because I looked like some idiot who couldn't control their dog.  That kind of reaction to such simple handling is a serious red flag, and probably part of the reason he ended up in the shelter to begin with.  I couldn't risk it. I brought him back to the foster.  Goodness forbid if someone else starts to take him home like I did, and is not as dog savvy, and ends up getting nailed by him.  I told them what he had done and why I had to bring him back, but I don't think they took it seriously.  He was the last dog on my list in the area to check.  Another 15 dogs down. All I keep getting around here is heartworm positive and severe behavior issues dogs.  That disqualifies them as service dogs flat out.  I can't afford to put money out to try another puppy.  Going to curl up and cry away my frustrations.