I have to say this again...but I will put it in a different way. If my parents, at 19, had asked me to cosign something, that they promised to pay themselves, I never would have questioned it, I just would have signed. I probably would have done the same at 40 as well...but that is only because I would have known that barring some sort of serious disaster, they would have paid. I sure would have known better even at 25 what I was getting myself into than I would have known at 19.
OP, tell your husband to open his own bank account and keep his money there, and then decide how to proceed with this mess. You are right, if the property is really worth 100k, then even in a fire sale the bank is going to get their money back.