Zero no Shiniki

Kanazaki Rei is an avid gamer, but his debts are mounting up as the expensive medical treatments for his sister don't come cheap… in a fit of desperation he follows a link in a mysterious email promising help only to find him transported into the dangerous world of his favourite videogame. For each boss he kills he earns money, keeping his sister safe, but he risks death with each confrontation!

(Source: Titan Manga)