I'm going to assume yes. They can technically do that, but I don't understand why they would. MSRP (of $500) is just a suggestion from the manufacturer (i.e. Microsoft in this case), but the when a product is bought from a manufacturer, the purchaser (in this case Walmart) already bought the unit from MS and then they can price accordingly to gain some profit. That is why there is a "unit sold" and "unit shipped" numbers.
Did you look at Walmart's website? If so, you have to make sure that it's "Shipped and sold by Walmart". Walmart has a marketplace feature like amazon. You have to see who it's being sold from. Usually, big named stores will keep items at MSRP, smaller mom and pop shops (and ebay sellers) sell them at any price they want.