William Twining is blessed with both brains and wealth. He attends a prestigious school and lives in a huge mansion with his uncle and butler. But when his uncle's business endeavours fail, the Twining family is left bankrupt.

While searching in a storeroom of his home for objects to put up for sale, William stumbles upon a secret room. This room contains a magical seal, with which he unintentionally summons a demon. This demon, Dantalion, tells William that he wields sole authority in selecting the "substitute ruler" of the demon world! So naturally, the candidates for the throne will pursue the bewildered and annoyed young man...

Do you have a link to the manga?

Going off the pic above it looks like it's on mangahere.com.