Heat and soften the adhesive label with a hair dryer set on high heat, held 10 to 12 inches away from the leather.
Scrape the majority of the adhesive away gently with a fingernail or playing card.
Scrub the remainder of the adhesive residue away with a microfiber cloth moistened slightly (not soaked) with warm water and leather soap (used according to the manufacturer's instructions). As you scrub, work in the direction of the run of the leather's grain. Never scrub in circles. If the adhesive remains stubborn, switch to an old toothbrush used in the same way as the microfiber cloth.