I guess this guy is in Megumin's Explosion Cult?

Maybe. It's magic, we don't know what the hell happens to the money after he spends it. It could be distributing itself randomly across the world for all we know.

you know, if MC drops money into his status screen in exchange for stuff, isn't he reducing the supply of minted currency?

That kind and sympathetic paw on the head. My dog does that to me when i lay on the floor too.

And he wants to learn them without facing difficult foes... that's a bummer... just go and learn some skills will ya..