If a mistake was made, it will be caught by BMW FS and it will need to be corrected anyway. The deal is not final until accepted by FS, even after delivery, so you should address this up front if you are certain that a mistake was made. We have a few CA's on the site who may chime in here.
Also, they should be calculating the residual based upon MSRP (rather than negotiated price), correct? This could account for the difference.
If you haven't already done so, check these resources:
http://www.leaseguide.com/lease08.htm
http://www.leaseguide.com/calc.htm