In the novel didn't Fanucci go to Vito's apartment? I think that's where the haggling over the money took place. He went looking for "his" money, not Vito going to see Fanucci. Vito then made sure people saw Fanucci leave the Corleone apartment alive and well and HAPPY, no sign that they had argued. Vito then made his way roof to roof to a building across the street from Fanucci's building. He went down the stairs, crossed the street and waited in the entranceway of Fanucci's building. He killed him just inside the entrance to his apartment building.

The white suit I think was a way of flaunting his supposed status. In those days, wearing white and keeping it clean, showed you didn't do manual work. Average people didn't do laundry like we do today.

An interesting point from the novel....Sonny witnesses his father killing Fanucci.