Act 4, scene 3 begins with Cassius accusing Brutus of wronging him for punishing a corrupt soldier Cassius had supported. Brutus defends his actions by saying that supporting justice is more important than patronage. Indeed, it was in the name of justice that they conspired to kill Julius Caesar. As the two Romans exchange insults, Brutus expresses his displeasure and disappointment with Cassius. He calls out Cassius for being corrupt himself:
Let me tell you, Cassius, you yourself
Are much condemned to have an itching palm,
To sell and mart your offices for gold
To undeservers. (4.3.9-12)
In saying this, Brutus is insinuating that Cassius could be just as guilty as the man Brutus just had executed. Brutus presents himself as an honest and virtuous man and Cassius as self-serving and greedy.
Brutus also takes this opportunity to remind Cassius that his requests for gold were denied. Brutus goes on to say that he cannot pay his military expenses himself because he refuses to raise money through corrupt and unethical means. He insinuates that Cassius has stolen money from poor Romans and is hoarding it all for himself.
Cassius is eventually able to calm Brutus down, and Brutus admits that his anger towards Cassius is likely misplaced. He is upset over the untimely death of Portia and is taking his grief out on his co-conspirator. None of this is to say that Brutus did not have any fair points in his attack on Cassius. This whole exchange presents Brutus as a virtuous idealist and Cassius as a shrewd and practical politician who is not bound by ethical considerations.