Do Not Open This Box

An aged inventor and his wife come into possession of a box marked "Do not open this box." The wife opens it and finds it empty. The next day a man comes to claim the box and the lady makes up a story about losing the box. The man begins giving them nice things in exchange for eventually getting the box back. The couple begin living the sweet life thanks to the stranger but when they finally give the box back, the man tells them that it contained a human soul - since the box was opened the soul has escaped and now the man needs a replacement.