Ah I see we have "dumb weak protagonist" here at work, a staple of Japan. Dude does not consider any other possible options such as Mashiro being used and threatened as a pawn, or that someone else is behind the current killings, etc. Does not attempt to pry secrets from Mashiro via threats, doesn't even think of that. Does not question where the money came from or use it to arm, armor, or prepare himself in any way - thus the money is in effect an already forgotten plot tool.
Of course, all that would require a main character with half a brain or isn't spineless but again, Japan seems to love their weak pathetic MC's who are only strong enough to participate in the story as needed for the author. Ones that don't think much.
Of course there's the OP Japanese MC but this wouldn't work with someone OP with powers so eh.
This is gonna be a boring ride with the story leading the weak MC around as essentially "our, the reader"s camera, our view into the story.