They wanted to break his organization of the (successful) unionization of the Brotherhood of Sleeping Car Porters. He declined and was able to secure rights and protections for the Blacks working on the trains.

Money has been around fooorrrreeeeevvvveeerrrr, as has opportunity to be corrupted.

Martin organized the March on Washington with Randolph, who was on the stage with him. In fact the threat of a March on Washington is something Randolph developed and used to push through Executive Orders to desegregate the military and government/public service position to create a ton of jobs for Black folks.

Malcolm was part of the NOI and was the main figure of their brand behind THEM. It would stand to reason that had he been corruptable he would have had immense opportunities to fall in line and not rock the boat and therefore rock him money, or his life. I think it is evident where his and King's integrity lied as they thoroughly practiced what they preached on the point of Civil Rights. They weren't perfect but they were personally flawed, and didn't allow those flaws to bring down the movement.

That's why these guys are heroes, because they DID get faced with the temptations and decided to keep pushing forward.