I have this idea that Mario Calderon wrote that letter already planning to use it as a trump card.
Like the same day he wrote this, Armando tells him how special the second night was. It was already obvious to him that getting rid of betty wouldn't be easy. Even though in the call he tells Armando to destroy it, I don't doubt he saved a copy.
For me that day it became clear to Mario even if everything with the embargo went well, Armando was going to keep Betty with him. And to Mario this is bad news.












