Call customer service and have them straighten it out. Due to the error, they should also cancel the booking and unblock the days. Because this is such short notice, unblocking the dates may not be an advantage, since so many people book in advance.
I would double check listed rates on the app right now and make sure that they list $128. If it does, and the people show up, I would insist that Airbnb make good on the amount, and pay you the full $256 themselves, since the app was the problem.
Good Luck