I'm trying to find a short manga I read some time ago but I can't remember the title. It's about 20 chapters long.
There is this boy who is bullied and is about to commit suicide jumping from a building, but a demon lord or something like that stops him and offers him a deal. The demon will give his family much much money to make their lives better, in exchange the boy has to keep living for a year, but during that year some sort of power will be growing in his heart, when the year ends one of the demon's daughters will eat his heart and become very powerful.
The two daughters start living with the boy the next day to protect him from other demons/monsters trying to steal his heart.