Just spinning off theories but
If the MC were to break a finger on each hand, then he wouldn't be able to play paper normally, since it the condition that they must both be able to play the game wouldn't be satisfied.
He also doesn't need to bandage all of his fingers, just one (preferably the ring or pinky, maybe?) if he doesn't want to be caught unguarded.
Also, whenever the devil appears, he could just suddenly clench his hand and make a fist, declaring his win. The devil can't be the only one able to do that. If the devil declares him a cheater, he could just bring up the previously discussed conditions as well as the devil's previous examples as well.
I haven't read the next chapters yet, so I could be very wrong about things.
