Kono Sekai no Mukou de

The protagonist of this story starts having been left behind by his parents with an enormous amount of debt to some rather nasty collectors. Just when he thinks his life is over, he meets the proprietor of a shop known as the 'Regret Store'. She tells him she is a being of the world after death and offers him a contract, for which he will receive a chance to turn his life around in exchange for performing a task of her choice. This task is to 'save' a girl with only three months to live...