Not a retailer, but Bob Crandall, CEO of American Airlines in the 90s, regularly told Boeing or McDonnell-Douglas what price he would pay for a plane.
Of course, he was ordering 300 at the time, but he always signed contracts for aircraft that were below discounted prices.
Two things to note about this example. First, they were competing against a subsidized competitor (Airbus) in a very small marketplace. Second, did Boeing or McDonnell-Douglass accept terms that put their businesses in peril? If Boeing didn't sell to American Airlines, would most of their business dry up?