PM: Well I’m going to need some kind of discount. How about 30%?
Me: As it says on our website, we don’t discount.
PM: But I’m buying 400 seats!
Me: Yes, and we already provide a nice discount for bulk orders, which is already included on the invoice and documented on the website.
I.e. he was wrong to say 'we don't discount'. The answer is actually - 'yes, I've already applied your discount. Because you are buying so many seats, you get our discount of x%. If you bought another y, you'd get z%'. Sounds like it could have been communicated better.