If the deal is truly a loan + obligation that increases the odds of Sandro staying. If we would earn 70m through a single sale there aren't a lof of reason's to delay paying Costa's fee.
So what ? It is still a deferred payment over 2 years with multiple installments. If we had paid €50M at once this year, then that would be a different question altogether. Paying an extra €10M the following year isn't going to make or break us.