So the story is he killed the demon king, but got returned to the time when he was a child. He started training and building a reputation to become a hero. He digs behind a dungeon and just goes straight into the treasure room, so he could fund his career as a hero.
If I remember correctly, he also has a friend who is lazy but crazy strong. I remember in later chapters, they visit the demon country and contact the demon king, who is actually a demon queen. The friend has a sword that was used to slay the demon king before her. I remember that the new demon king is cloaked but has trademark black wings.
Note: not isekai manga. The protagonist just came back to the time when he's still young.