Tatoe Hai ni Nattemo

For his beloved younger sister's treatment, Shinomiya Ryouma is working frantically to raise a billion yen while going to high school. One day, a mysterious woman asks him, "Can you put your life on the line for what you desire?" and after receiving a ticket from her, an accident befalls him! When he wakes up, he finds he's turned into a girl and what's more, he's participating in a "game" where a win means he can reverse his fate but a loss means he'll die instantly!

(Source: MangaHelpers)