In Ronaldo's case also, he is not getting the entire amount upfront. $250 million will be paid over several installments, during his stay with Al Nassr. And then there is another payment for $250 million (so the total goes to $500 million), which is linked to Ronaldo being a part of the Saudi team, which will bid to host the 2030 World Cup. Usually the bid team includes only citizens of that particular country and that is the reason why there is speculation on Ronaldo accepting citizenship from Saudi Arabia. But we need to remember here that dual citizenship is not recognized in Saudi Arabia and therefore if he gets the Saudi passport, then Ronaldo need to forfeit the Portuguese nationality.
Even if Ronaldo gets Saudi citizenship, what would that mean for the football? Nothing. Even if he could, he still wouldn't be able to play for the Saudi national team because he played for Portugal. I really don't see the point in him getting their citizenship because as dar as I know, it's not forbidden for a foreigner to be a part of the group that will bid to host the 2030 World Cup.