Billion Girl

From ShoujoMagic:

For heroine Ooba Kanoko, life isn't much fun. Her family runs a small restaurant, where she slaves and is horribly mistreated by her two sisters, parents, and grandfather. (Not to mention the fact that her name said aloud as "Oobaka no ko" = "stupid kid"!) Poor Kanoko cooks, cleans, and takes the blame when things go wrong. However, her life takes an unusual turn when her family suddenly loses what little money they had, and everything they own is about to be repossessed -- along comes a ragged looking, tall stranger with an interesting proposition that will turn Kanoko into a billion dollar Cinderella! At a price to Kanoko...