-Cinna's request makes Cassius ask him to take an incriminating letter to Brutus.
-Cinna's request makes Cassius trust him to return to Pompey's Theatre and meet Cassius there.
The instructions were clearly given to Cinna, not Casca, and Caesar is not mentioned in the passage. The conversation ends with Cinna accepting the order to bestow the papers and Cassius telling him to "repair to Pompey's Theatre" after doing that.