Microsoft has a directly competing product, so I don't see why they would ever want to sell Wunderlust. They would essentially be creating their own competitor, in return for a fraction of the money they had paid to acquire Wunderlist earlier.
then again I doubt that the microsoft empire stands or falls with a to-do list app and it might just be good PR to sell it back instead of letting a well-liked product rot given that they're on an alleged "we love the community" campaign now.
Good PR--sure. $100M worth of PR? I don't think so.
Not to mention that even the full purchase price of $100-$200m doesn’t make the Microsoft CEO even raise one eyebrow one millimeter.