The Adventure of the Christmas Pudding, by Agatha Christie (epub/Kindle/pdf)
Poirot is asked by a Mr Jesmond, who is acting as an intermediary to an eastern prince, to help that unfortunate young man with a problem he is having. The prince is due soon to be married to a cousin.
On his way to be married, he met an attractive but obviously dubious young woman, and rashly decided to have a last fling with her. The prince brought several expensive jewels with him to London for resetting by Cartier and one of them, a fabulous ruby, was stolen by the young woman. If it cannot be retrieved, a scandal will ensue and because of this, the police cannot be involved.