I am wondering if a damp "color catcher sheet" will do the job. As far as using cleaners on leather you have to treat it afterwards with a leather conditioner or the leather will dry out & crack.
I googled jeans dye/leather seats, but there are so many suggestions it's hard to say which one will work best. I got everything from Dawn dish detergent to using Turtle wax.
The dye used in jeans never fully dries, hence it rubs off on just about anything.