Is something like the Raya deal be possible? Kane signs a 1-year extension with Spurs. We take him on loan this season for a nominal fee with an obligation to buy next summer for £80-90M.
It would work for everyone, Spurs get a fee for their best player who could leave for free next year and we get Kane a year early while still getting round our FFP troubles.
Obligations to buy don't go around FFP. If there is an obligation to buy then the player is considered permanently transferred. What you described is only interesting for United from a cash flow point of view, if that's an issue in the first place.