Hi everyone,
AirBNB wants people listing their homes or income properties to 1) learn how to use their site before listing properties, 2) only offer units on days for which they are available and for prices at which they are willing to offer them, and 3) honour their commitments.
It is unfair for people looking to find a place to stay who select your unit to have you cancel on them because you wanted to charge more for certain dates than you advertised, or because your place wasn't available on those dates. It is also unfair to AirBNB because it costs them transaction fees to accept funds and refund, and because their customers end up having a bad experience. It is even unfair in some cases to other people listing on the site who listed a similar property at a higher price but are willing and able to honour their commitment - if a user who would have booked with the commited lister ends up booking with someone else at a lower price who later cancels on them, they may choose not to use AirBNB at all and the commited lister might not rent their place out.
If you realize that you charged too little for your place only after someone books it, you simply do not have the option of charging more - you either have to honour your commitment or you get nothing for those dates (and a strike against you).
Although you might be trying to help your guests by offering a special (lower) rate, it is also a violation of your agreement with AirBNB for you to circumvent their system. They want to keep their offer and acceptance system as simple as possible.
If you list a place for less or more money than you wanted to and someone books it, your best option is to honour the commitment and adjust price structure and availability for the future.