Hi,
1. Clean the leather first (this rmeoves excess grease from the surface).
2. Apply degreaser (this soaks into the leather and then absorbs the grease out), once dry (roughly 20mins) it will have removed the grease, or some of it.
3. Brush it off and clean the leather again. If theres still some grease re-apply.
After removing the grease you may need to restore the colour, which mentioned above increases the price of the job.
When quoting for grease removal, always add on a bit extra as most times you need to restore the colour.