You can find a holo-tape under Seagrave bed in his room. You use that to either tell Danvers Holmes is doing bussiness with the slavers, or that Bannon is trying to blackmail Segrave.
This is just persnoal prefence, but I would side with Bannon as he has a copy of the Deathclaw schematic, you get a 10% discount with him, but a 10% increase from Segrave. But if you do the Rivet city optional quest for the wasteland servivor guide, you get an 10% discount anyways. So basicly it's like you are getting normal prices from Seagrave.