OK, I've made this mistake 2,000? More times? Options: Eraser or a clean finger to remove the film of the glue, Deglaze (yes they make leather deglazing) the entire piece. Alcohol will do the same job in a pinch. Soak it in alcohol and it will dry in a minute or two. If the spot still remains, even as a darker shades spot....dye the bastard black. About two heavy coats and you'll never see it. Last but not least....add tooling to cover it. This is my last resort but it works to add a little fill in or texture to cover. Sometimes this dresses a piece up a little more anyway.