You don't negotiate a money factor. You negotiate a purchase price, BMW FS sets the promotions each month, including money factor and residual rate. These can change each month with the new program. When you put in your credit application, you're locked in for 60 days, as stated above. If that expires, you have to put in new credit app and lock in new rates. If the promotion gets better by the time of delivery, you can also use the new, better rate even if your rate lock hasn't expired.
Did your payment, due at signing, or residual change from the price you were quoted when you negotiated the deal? That's really all that matters.