D-Limonene in water with alcohol and a surfactant. Spray on, leave for a while, wipe off, repeat until clean. This usually works for most things. If it is dog stuff on your leather, it might work.
I use a 16oz spray bottle, 3 Tbsp each of D-Limonene and rubbing alcohol and a couple of drops of dishwashing soap. I increase the Limonene if needed. It is a solvent so I wear gloves when making the concoction but the spray is harmless, at least for me and my dog. I use food grade everything.