The Magic Book is a Danish fairy tale collected by Evald Tang Kristensen in Eventyr fra Jylland ("Adventures From Jutland.")
The boy told him to sell the dog he would find by the door the next day, but be sure to take the strap back.
The boy told him to sell a cow that would appear the next day, and to the king, but he must take its halter and come back by the forest.
The cow appeared, and the man sold it, but when the butcher went to kill it, it turned to a dove and flew off.
But the princess and the boy fell through a tunnel there, to a golden castle, and when the king opened the tower for the funeral, there were no bodies.