At this point I am not sure of answer. Since it is already dyed it will likely show a difference! I had a fellow tell me one time that the old style typewriter erasers would take off contact cement. I realize a typewriter eraser is before your time, but you still can get them! The core is the eraser and it is fairly firm. However not sure it would work on super glue as it penetrates a porous surface such as leather! You might be able to scrape the area and get it off but it will show a difference. Don't ask how I know that!!
When it first got on there you might have been able to remove it with a thinner or acetone but you would have still had to wash the whole sheath, as it will change the way leather takes dye.
Maybe some others will chime in with other ideas. How did you get the Cyanoacrylates AKA superglue on the sheath??