When I worked as a
travel agent we were told only the amount that was paid for was guaranteed. That's why they are only honoring the one night deposit. Only if the amount was paid in full is that price guaranteed for the entire vacation. It happens all the time in the travel industry, companies give you an incorrect quote and you have to go back and tell the client it's more (wow, that makes people happy), some companies just change their package prices (like Amtrak) and people end up having to pay more. It's not unusual or illegal.
Sorry, but you should probably be prepared.