Try to get a giant bankroll that he and other FIFA officials could embezzle, or else get kickbacks from the buyers to enrich themselves?

I can’t think of any plausible, legitimate reason for FIFA to mortgage future revenue like this. The only situation in which a move like that can make sense is if there’s a huge near term cash flow problem, such that you absolutely need immediate money on hand and are forced into it. Other than that, though, if you are offering a price for which there are buyers, then by definition it’s a bad deal for you. No venture capitalist would buy the future revenues unless they were clearly worth more than the amount they are paying for them. This is not some complex financial deal or one where there’s a lot of future uncertainty, like buying a company. It’s a straight purchase of future cash for present cash, with a very stable and easily projected value for the future revenue.