When purchasing an enchanted book from a cured master librarian villager (Mending for a discounted price of 7 emeralds and 1 book), he gives me a different enchanted book that he also sells (Projectile Protection 3 at a marked up price of 41 emeralds and 1 book). When I'm making the purchase for Mending, I'm clicking on the Mending item in his inventory, then shift-clicking from the box on the right to complete the purchase. I'm then left with several Projectile Protection 3 books.