Please please do not use milk or any other household products especially baby wipes to clean your leather (baby wipes do a lot of damage to the finish on leather)
This problem is probably not going to be fixed simply by cleaning. It sounds as though you may have an aniline leather that is absorbent and so has left a bad stain.
You need to take professional advice from people like LTT at
http://www.LTTsolutions.net as they are leather consultants with a great deal of knowledge.