@Michel682,
If there is already an confirmed reservation, you can change it and put the right price in it:
https://www.airbnb.com/help/article/50/as-a-host--how-do-i-make-a-change-to-a-confirmed-reservation Do not cancel the reservation !
A special offer can only be sent to a guest contacting you by inquiry, not if it is a request
The guest has to pay to Airbnb at time of booking. After the guest checked-in, you will be payed by Airbnb (except when you are a new host and it is your first reservation)
You can contact Airbnb by Airbnb Help: contact
The phonenumber is also there.
Please note the HELP system can answer all your questions, by putting in the right search keywords like: special offer, payout, contact etc..
Hope this helps,
Emiel