18+ Kokugensou wo Item Cheat de Ikinuku the Comic

This is the story about a bashful and inconspicuous high school student surviving a harsh fantasy world with wisdom, bravery, and very few skills. Apparently he wanted to become a hero with a magic sword, but unfortunately for him, he possesses no swordsmanship or talent for magic. The only thing he can count on is his half-baked knowledge he acquired previously from modern Japan. Now, in order to survive, he's forced to convert that knowledge of his into money. While doing so, the main protagonist will rise into becoming a secretary assistant, trade company owner who manages slave girls, knight, and a hero.

(Source: Kakuyomu, translated)