I wonder if the "deal" was going to be just "we'll run your US infrastructure (and keep data) in the US, on Azure, and we split the money" rather than a buyout ? That would make more sense to me. So more like "Microsoft will invest X billion for a Y% of TikTok and run its US operations". You could satisfy the US government that Microsoft was guaranteeing the data would not be shipped to China.
The only way to make that arrangement work would be to give Microsoft access to the source and ability/responsibility to review the source/releases. Not exactly a great Setup for either party.