This is one of those places in Julius Caesar where Shakespeare does not give enough information to form a definitive answer to the question of what really happened. We do know a few things from Act IV, scenes ii and iii.
We know (1) Lucius Pella of Sardis did take bribes and Brutus accused him. We know (2) Cassius asserted in a letter that Pella's offence should be overlooked. His argument was that "In such a time as this it is not meet / That every [small] offence should be" scrutinized and condemned.
We know (3) Brutus accuses Cassius of being greedy by selling his influence, which can certainly be interpreted as taking bribes, to which Cassius responds in a sincere sounding cry of protest, although it can be suggested that Pella paid money Cassius to write the letter trying to protect Pella.
We know (4) Brutus asked Cassius for an advance of money with which to pay his legions of soldiers and, according to Brutus, his request was denied by Cassius, whereas Cassius claims some messenger bungled the message and got it wrong. We know (5) that after the argument takes a potentially violent turn with Cassius offering his dagger and bared chest to Brutus, they forgive each other and confide in each other. We also know from scene ii (6) that Brutus believes Cassius
"Hath given me some worthy cause to wish
Things done, undone:.."
We do not know if Cassius did takes bribes from Pella for writing the letter, or for any one else for any other purpose. We do not know if Cassius ever did advance Brutus the money he needed. Based on the inferences that can be drawn, one has to draw one's own conclusion. It seems to me that since they forgive each other and Brutus confides in Cassius about his wife Portia's death, that Brutus has come to believe that Cassius is, in fact, innocent of taking money for the letter supporting Pella or anything else.