Krazykats that dude is clearly using you to buy down the competitors offer. It's his job to do that as much as its yours to sell.
Ultimately you've got to stand your ground, call him on it, and let him know you're in this to do business, not to be a quote machine. But that being said, you've got to be in a position to tell him to F off (when you literally don't need his business in order to win), and not in a position of desperation (which you probably are, and he can sense from a mile away).
Great advice!
I'm almost there to be able to "train" bad customers to be more loyal or go elsewhere, because I have a few. They are the most time consuming and always looking to me to make up for others mistakes.
Love my job though, so good with the bad and that sort of thing.