Hi,
If you clean an aniline leather then you will most likely darken it as it cleans. This is because the leather is absrobant. However, do not worry, after a while the leather will dry and go back to normal.
It can be very hard to achieve satisfactory results with aniline because it absrobs a lot of dirt, which is then very difficult to remove.
What you can do, and it is very sucessful, is darken the leather using a product that will dye the leather. An easy solution and somthing to use that requires no training is Grison Leather Re-Colouring Balm. Just rub it on, the leather absorbs it, buff the leather over and jobs done!
It may be best to go to the customers house first and do some tests to see if it actually is an aniline leather first.