Ask for them to honor the price on a similar unit, since you already applied. You applied for a specific unit and a specific price. I don’t agree with their strategy here, but they are doing this because they want you to move-in. Not because they want your $400.

Be nice, but firm. Ask them for a complete refund immediately, or to apply the price to a similar unit. If they say no or cannot give you a firm answer, follow up by asking for the Regional Manager’s contact info.

As always, remember to be nice. No one wants to help an asshole. Especially when they are going to see you every month when you pay rent.