From what I gathered from the various reports, it's no longer about the total sum of money that's at stake, but more about the terms of payment. Wolfsburg are under UEFA FFP investigation too, and it looks like they want to spread the payment over several seasons rather than paying it all at once for FFP reasons. Maybe Chelsea, for FFP reasons, don't agree.
"Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!" - Red Queen, Alice - Through the Looking-Glass