Len, if it is a pigmented leather, and you can remove the filling, via a zip, you may be lucky.
Remove the cushion from the suite, if possible, undo the zip and remove the filling, be it dacron or whatever.
Turn the cushion inside out.
The next steps are best carried out, outside, or in a well ventilated area.
I always carry a small table to work on.
Heat the offending area with a hair dryer.
(This will soften the oils, and allow for easier transfer).
Spray on Leather Master Degreaser.
(This will go to a white powder nearly immediately).
Leave it on for about fifteen minutes, vacuum, wipe with a white towel, inspect and repeat.
It is very time consuming and should be charged for accordingly.
The reason for turning inside out, you have probably guessed, allows the offending material to be drawn to the back, instead of continually to the top face of the cushion.
CAUTION after replacing the filling, put your finger inside the cushion under the zipper. As you pull the zipper along, your finger will prevent any filling from getting caught in the zipper, which could possibly cause more damage.
Now get off to work you lazy blighter

I'm just sitting back, sucking on a rum 'n coke, 5:30pm.
Cheers,
Shorty.