Fairness and limits in business go hand-in-hand, though they may initially seem to work against each other. This "push back" email, done after a few rounds of negotiations, finds the perfect middle ground between treating customers fairly while staying true to your company's limits:
Dear…
As you know, my priority is to find a mutually beneficial solution to establish a solid lasting relationship between our companies.
As much as I would love to offer you a lower price, I am limited by our company's pricing policy. The [ X]% discount I managed to get you is actually quite an achievement given that… I do realize and appreciate that you are looking at it from a different perspective. However, we need to meet somewhere in the middle. I hope you could reconsider our discounted proposals so that we can move forward with [benefits].