The problem with persuasion in business is that when someone like this person turns around and tells you they can't use you there's no way to tell whether he's actually bluffing and does have the necessary budget.
If he doesn't and you don't negotiate then you'll lose the deal but if you do then he'll feel he can do it in the future too.
What really matters isn't whether or not you're able to negotiate with people, what matters is the quantity of work coming your way, if you have more work coming your way than you can handle you can afford to to walk away(which may cause them to buy at the original quote price), if you don't then you have to accept their lower offers. The principle is similar in
pua, willingness to walk away..
Have you read "How to win friends and influence people?" it covers quite a bit on sales, it's a classic.
I also recommend "The greatest salesman in the world". Excellent book.
Edit: Just checked I had the name of the second book right, it has 167 5 star reviews on Amazon too.. Other people agree with my opinion of it =P
Amazon.com: The Greatest Salesman in the World (9780553277579): Og Mandino: Books