Some countries, like Portugal have a few club owners, but a lot of club presidents, voted by the fee paying fans. So, there is an incentive to "misplace" some parts of the huge fees on transfers.
Later ot became known that the whole operation cost 128 millions, and was bound to happen exclusively if Mendes was involved.
So, the player had to be "convinced" (it was clear he didnt want to leave so soon). In the Benfica accounting investigation (which led to the presidents arrest), it became clear that only around 50 million really got to the club. The rest was divided in "fees and commissions" not easily determined.