I think they just plan to kill Brody after he's given them all the information he can/helps them capture Abu Nazir. I remember Estes saying earlier this season that Brody was pretty much the end of his career because he's fucked either way. If he tells Walden that Brody was turned, then he let someone infiltrate the highest levels of the US. If he doesn't, he's just buying himself more time because if Brody does help them, their agreement was some sort of immunity and a terrorist essentially gets to walk free. This way Estes gets to have it both ways with Abu Nazir dead and nobody knowing about Brody.