Microsoft is a publicly held company and the Bungie asset is absolutely a financially significant contributor to current value and future earnings. All of which is to say that Microsoft cannot just "let Bungie go" like it was a three date relationship that didn't work out to everyone's satisfaction.
Is there someone who would buy Bungie at market value that MS would be willing to sell to? Do the Bungie guys have enough money of their own to buy the company back with possibly an agreement to provide "x" more installments of Halo as part of the deal?
I dunno, neither of those feel all that likely to me.
Now, some kind of quasi-independance where MS still owns most/all of the company and yet Bungie gets to breathe a bit freer? I suppose I could see that happening.
If there is any truth to the rumor at all it has to be something along these lines. More a realignment of their working relationship than a sale.