If Wilder invokes the re-match will have to beat Fury and then have to beat Joshua in order to become undisputed and unified heavyweight champion of the world.
If he steps aside by taking $20 or so then allows Fury and Joshua to battle it then fights just once in order to have a chance to unify the belts and become champion again - that makes sense too.
Do you think the promoters will be willing to give Wilder $20 million to step aside, i have no idea about the money they are talking about but it is a really huge amount for not fighting if the amount is around those figures. I bet the Wilder camp does not want to take the immediate rematch but with the ego Wilder is having he would not let that go either. If Wilder takes the money and let Joshua and Fury fight then there is no guarantee that Wilder will be getting the fight again.