The explanation sort of makes sense, or maybe someone took it off for better visibility on a road test, but it might be smarter for the dealer to get another sticker from Nissan or find the existing one and pull the Cube off the lot until that time.
Per Wikipedia, the dealer could be subject to a fine and jail time if the Monroney sticker is missing. Only the consumer is supposed to remove it. Maybe the dealer was shocked that someone may call them on it?
I suppose you could make getting the sticker part of the deal (a "we owe" agreement). You do have to wonder if the dealer is running some kind of scam or just a loosely run store that can't keep up with the paperwork. Was the Cube sold to someone else and the deal was unwound before (or after?) the title went through? Was it used as a loaner or demo and then put back on the lot? Or was the Monroney simply stashed in the glovebox and got accidentally tossed?