@Simkin something like that does exist in the novel version as a side story after the end of the main story. Hopefully those chapters will be included in the manhwa adaption as well.
He needs money as incentive to intervene in a foreign land to which he has no affection nor allegiance. If he wanted to do charity work regarding gates he'd focus his efforts on his own home/ where all his family and friends reside. He doesn't owe Japan anything. They're the ones asking him to come in when he could be doing things more relevant to his personal interests / those of his own family, friends, and nation.
(Money is always useful btw cause even if you don't know exactly what you want at the moment. As long as you have enough money then you can generally use it to get what you want when you finally do make up your mind/ when a whim strikes you.)
That evil russian guy is not stereotipical enough for me. I would've prefered more vodka and balalaika, and a pet bear of course. Evil japanese guy at least had a katana