I'm going to scam my roommate.
Why do you want to scam your roommate?
He's asking for a favor and I am going to pretend to do it.
It is not good manners to play tricks on your roommate. You will damage your relationship and they will no longer trust you.
Fuck him, he can't expect my help at any cost.
If you don't want to help him, then tell him you can't. Tricking him makes the situation a lot worse as he will think he can trust you, but he can't.