Amberser wasn't exactly what you'd call an old friend with this person; rather, they had an argument about alchemy over a decade ago. Amberser then discovered that amongst humans, there were those more terrifying than his skeletal face.
It wasn't that he was ugly—on the contrary, Gustavo Flint had handsome features and was a very charismatic older man.
But his dark skin and the imposing aura he carried when expressionless were enough to terrify people. There were rumors about a certain alchemist that got a heart attack simply from being stared at by him without a word.
Moreover, Gustavo Flint's strict, almost severe, demands in alchemy made many alchemists who had worked under him tremble at the sound of his footsteps.
Many said behind his back that Gustavo Flint was a Lich in human skin, which was why he had an Aura of Fear.
When Amberser quoted twenty million, Gustavo Flint decisively responded, "Five hundred thousand."
"Eighteen million."
"Eight hundred thousand."