This short novel by H. Beam Piper was originally published in two parts in Astounding Science Fiction, February and March of 1955 and was also published in the 1981 collection, "Paratime."
The heroes of this story, Verken Vall, and his wife, Hadron Dalla, are ready to embark on a long-overdue vacation, when the Paratime Police send them both to investigate rumors of slavery in the Esaron Sector. Like many of Piper's stories, this is a police procedural, set in the world of the future.