Van Buren and Laszlo
Despite recognition of Laszlo's talent, Van Buren was envious and resentful of him. He felt the need to remind him that he was dependant on his largesse. His petty action at the dinner when he tosses the coin at him was an indicator of his true feelings. He needed to humiliate him. Of course, this all culminates in a shocking act( no spoilers) later on in film. The fact that Laszlo was Jewish and an immigrant made Van Buren feel even more superior.
This reminds of the relationship between Richard Nixon and Henry Kissinger. Nixon resented Kissinger even though he needed him as his foreign policy advisor. Kissinger knew that he had to suck up to him to stay in White House.