What indication are we given that Cassius is upset with Brutus before Cassius even arrives?

Cassius is upset that Brutus publicly disgraced a friend of his for taking bribes from the Sardians. Apparently Cassius had sent several letter to Brutus urging for the man's release, but Brutus refused. Brutus is furious that Cassius would even consider defending a man for taking bribes, arguing that Caesar was killed for exactly such behavior.