Is it permissible to keep money that was given for the purpose of buying publications, and then these publications were obtained for free?

The agent must return the money that was paid to him to purchase a specific item if he did not spend it for that purpose, as it is a trust (amanah). He is not entitled to keep it unless its owner willingly agrees. If he wishes to sell the publications to the principal after acquiring them himself, he must conclude a sales contract with him for a price they both agree upon.