You can try rubbing a little brown shoe polish onto the spot. Test on a hidden area first to see if you like the results. (The underside, back, or skirt flap if there is one.) Part of the beauty of leather furniture is the unique way it ages. Wear & tear, all that. I wouldn't panic. Just get used to it. Furniture should be used & enjoyed.
Look for a Tandy Leather store in your area (there's a store locator on their website, www.tandyleatherfactory.com ), they can sell you leather dye and also can give you advice on how to treat your sofa to prevent further damage.