From UEFA site.
Loan of a player from the lender club to the new club with an unconditional obligation to buy
a) The loan must be reflected by the lender club as a permanent transfer and the player’s registration rights must be derecognised from its intangible assets. The proceeds from the loan and from the future permanent transfer must be recognised from the inception of the loan agreement.
b) The direct costs of the loan and the future permanent transfer for the new club must be recognised by the new club in accordance with the accounting requirements for permanent acquisition of a player’s registration
Loan of a player from the lender club to the new club with a conditional obligation to buy
a) If a condition is considered to be virtually certain, then the player’s registration must be recognised by both clubs as a permanent transfer from the inception of the loan agreement.
b) If the fulfilment of a condition cannot be assessed with sufficient certainty to trigger the permanent transfer from the inception of the loan, then the player’s registration must be recognised first a
https://www.uefa.com/MultimediaFiles/Download/Tech/uefaorg/General/02/56/20/15/2562015_DOWNLOAD.pdf
I don't know why it's not a sale with deferred payment, but loan with obligation to buy is as good as permanent transfer for FFP accounting.