I'm not sure if it will harm the leather but I have always had good success with either the Goo Gone or Goof Off products. You could try it very lightly to make sure it isn't going to mess with the leather first.
I've used it once already at a friend's house and I liked it so much I got my own. The brush really works well and with some good scrubbing with the cleaner the ink should come out. Good luck.
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