Territory Management For The First Time, By The Adventurers Of Level 99

A high school student, Hayato Amakusa, was summoned to another world, Blue Earth, and became a great mage who saved the world with his companions. He was knighted for his achievement, and was assigned to the female paladin Valtrude, to help manage her territory. Ruined towns, ports that didn't pay taxes, dwarven villages that no longer produce iron... Hayato will uses magic, modern knowledge, and cheat-level financial power to tackle these problems! Until one year later, when he was sent back to Earth...

(Source: Futabanet, translated)