He went to a magical casino run by a devil and manage to clean house by cheating. Then he exchanges all his tokens for rare items but forgot to exchange some cash. He instantly pay it back using an alchemy exploit anyways
He doesn't 'lose' any money. He actually wins hugely. The problem was that he converted all his money into chips and the chips only convert into 'unsellable' items. He needs 1000 currency to buy the beginner alchemy set so that he can change his 'unsellable' items into trash items that are sellable for a shit ton like 'orihalcum waste'. He hands the hero back more money then he borrowed the same day he borrowed it. Though he DID shatter his expectations like 3 times along the way. Which is pretty much the MC's shtick from now on, not being 'hero material' but accomplishing amazing things through dirty methods.
Some heavy spoiler here, dont read it if you dont want to be spoiler
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The MC is weak.. so weak that even if he is level 50, his stats is actually an average level 30's stats. There is still no Hero (the cheat Hero) so far and MC is focusing on strengthening other.
MC won just because he exploit all his game knowledge and his skill moreover the MC actually "reject" the plot armor the world giving him to proceed the scenario like the actual game (you know, just like this Demon fight). Its funny that MC actually using some already mentioned weapon to defeat the strong enemy.
The latest WN is where the MC met the Prince of Light (the strongest person in the game) and actually punching him in the face. lol