There's no "contract" that I know of. You maybe mistaken with a deal Bungie had with Take 2 before the MS buyout. Take 2 bought something like 19.9% of the company to get Oni on PS2 and to get the distribution rights to Bungie stuff. When MS bought out Bungie, from what I know of they own them outright, Oni and Myth 3 got handed over to Take 2 as part of the deal to get Halo and Bungie.
And if you're thinking "what about Halo for PC and Mac...." that was also part of the deal. Halo coming to PC and Mac is more less MS appeasing PC and Mac gamers. The whole deal was pretty complicated. And Microsoft, shortly after the buyout, confirmed that Halo would be part of some lineup of MS Win games going Mac. Like Ages of Empires and some other stuff.
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