That part is not really clear. My understanding is that if he physically threatened you, you could file for a criminal case and he'll be in much deeper trouble than just not paying.
Specifically this part:
> Consideration cannot be illusory
A contract without consideration is void. If you promise to give someone X (that you own) if they give you Y (that they own), and you never give them X, then you don't own their Y, even if it's already in your possession.
You just (non-forcefully) stole their Y—same as if you e.g. asked to borrow someone's phone and then walked away with it.