I assume that you have been using the platform to communicate with the guests? In that case the price discussion will be there and proof of what is agreed. Remember that once they book, you then need to amend their reservation to the agreed price, and they need to accept it for you to receive the correct payment in due course.
@Isabel237 make sure to send an alteration request with the new total. Then your guest will need to accept it. That way, it will be reflected in your payout.