I have no idea of the numbers but I imagine a pretty easy repayment schedule could be designed.
It's basically a structured finance deal.
These things happen daily.
FSU has a pretty stable revenue stream to collateralize (BIG Conference revenue).
FSU would issue $500m (cost of ACC buyout) of notes to investors.
Future revenue streams (based on BIG conference payouts expectations) would payoff the notes (for example principle plus LIBOR) on a quarterly basis.
Honestly, I wouldn't doubt that more schools go this route in the future to pay for infrastructure improvements ($500m for stadium upgrades or something).
Instead of waiting for private donor money (that may never actually come).
FSU would be behind in usable conference revenue (since they would have to pay back the notes) but they would still be ahead of their ACC payouts for the next 13 years.
I doubt the Big Ten wants any part of the legal liability FSU would bring in breaking their GOR with the ACC.