What I have done in the past with other BMWs (although not Ms) is if a service is near, not do it but ask the price which enables me to do a service. Then I simply offer to the buyer, I can get a stamp in the book for them OR if they want to negotiate on the price then I will do that but not if I am going to service it. With a normal beemer obviously you're only talking a couple of hundred, the M is a bit more serious but same principle.
I have not yet had a buyer who doesn't feel the need to haggle, I think too many of them have seen Wheeler Dealers. So the easiest thing is to start off by giving yourself a (modest) amount of wiggle room, somewhere.