[Lord Syslox of Emyn Beverli]

If you read the exchange between the Mouth of Sauron and the Captains of the West you wil notice the Mouth is just trying to goad them into surrenderring.

Note especially when the Mouth gives the terms for suurender, Gandalf says something like "these are great terms for just one spy...bring forth the prisoner so we may have some surety on these terms." Then it says something to the effect that the Mouth was at a loss for words for a heartbeat. Obviously he couldn't bring the prisoner because he had escaped, so the Mouth covers up by saying "You crave surety, Sauron gives none." It actual says that Gandalf NOTICED the Mouth's lapse.

That may have influenced him in rejecting the terms, but im sure it was more the fact that he thought the quest was lost.