The manga might focus on the guy's efforts to raise one billion yen for a noble cause, like saving a community or achieving a scientific breakthrough that demands significant funds.
Maybe it's a tale where the guy has a huge debt and needs to come up with one billion yen to clear it. Or perhaps he has a grand dream that requires such a large sum of money to fulfill.
It could be a story of a guy striving to earn or obtain one billion yen through various challenges and adventures.