@Patricia91
Patricia, you probably won't get a lot of sympathy on the forum I am afraid. This is the sort of thing which trashes the Airbnb brand and builds suspicion into the minds of potential guests. This guest has no doubt made other plans that revolve around her stay with you, and with the click of a mouse on your part their plans are gone in a puff of smoke. They have to start all over again!
Before you signed this 'offer you could not refuse' you should have realised you had made a committment to someone else and cotacted both that 'reserved guest' that you accepted, and Airbnb and sort to have the guest and Airbnb work out an alternative to your accommodation.
As everyone else will say, this is not an extenuating circumstance Patricia and if you were the guest and had been treated the same way you would be rightfully indignant.
Your post was all about you and how you can reclaim your money, nothing about the guest you have let down, and I would be disappointed if Airbnb did release you from penalty.
As I said this is trashing the brand, and along with most others you won't get any sypathy from me!
Rob.