What does Brutus tell the servant to relay to Antony?

Brutus tells Antony’s servant that he will not be harmed if he comes to meet Brutus. 

Brutus really does believe he is doing Rome a favor by killing Caesar.  He thinks that Caesar’s ambition to be king makes him dangerous, and as long as Caesar is in power the Romans may as well be enslaved.  It is also important to Brutus not to kill anyone else besides Caesar.  He does not want them to be considered butchers, because he wants people to consider them heroes. 

After Brutus and the other conspirators have killed Caesar, Antony sends a servant in to find out if he can speak to Brutus.  He wants Brutus’s assurances that he will not be harmed.  Antony pledges himself to Brutus, through his servant's words.

If Brutus will vouchsafe that Antony
May safely come to him, and be resolved
How Caesar hath deserved to lie in death,
Mark Antony shall not love Caesar dead
So well as Brutus living; but will follow
The fortunes and affairs of noble Brutus
Thorough the hazards of this untrod state
With all true faith.(Act 3, Scene 1) 

Brutus accepts this pledge, and tells the servant to let Antony know that no harm will come to him if he comes before Brutus and the others.  This fits with his plan of not harming Antony, which Antony would not have known about. Antony would have assumed that Caesar’s allies would be killed with him. 

BRUTUS

Thy master is a wise and valiant Roman;
I never thought him worse.
Tell him, so please him come unto this place,
He shall be satisfied; and, by my honour,
Depart untouch'd. (Act 3, Scene 1)

Antony does come.  At first he is overcome with grief and sentiment at seeing Caesar’s body.  He recovers and shakes the hands of Brutus and all of the other conspirators.  He asks for permission to speak at Caesar’s funeral, and over Cassius’s objections Brutus consents.  The only condition is that Brutus speak first and they use the same pulpit.  Antony readily agrees.

Actually, Antony is being very cunning here.  He knows that if he is able to speak in front of the people, he can convince them to follow him and turn them against Brutus.  He easily convinces Brutus to give him what he wants through flattery and obeisance.  Once on the podium, Antony is a much more convincing speaker than Brutus, and Brutus and Cassius have to flee Rome.