I think under normal circumstances the sale isn't deferred to the end of the player's existing contract, so this is an odd case. However, it's certainly true in this case because the agreement wasn't just a deferred obligation, but includes a sell-on clause. I imagine this was a three part negotiation between Marseille, Everton and Maupay's agent, and he'd have already agreed to the terms that would follow the loan.
Maupay just told the French press that he expects to stay at Marseille, but supposedly they intend to sell him. If so, Everton should be paid the initial sale fee plus I think a 20% sell-on fee.