Victoria,
Why would you want to give someone a refund on the cleaning costs, especially before they even come? Do you know these Guests? I would never do any business, refunds, extensions, more Guests, or anything outside of the airbnb platform. You are just asking for trouble and never do any cash transactions with Guests as you will have no record of it and it is breaking the airbnb contractual agreement you have with them. You could easily loose your listing or at the very least be fined.
I don't understand why you would do this. I hope this works out.
Eloise at Happy Trails