I suspect if you told these 8 graders who “can’t subtract” you were going to give them $100 and ask them for $110 back you’d find they understand both subtraction and negative numbers.
Isn't this the actual core of the problem then? If a kid can do $100-$100 but can't do 20-8, the problem is he doesn't understand how to map the things in real life he knows, to the symbols you're showing him.