@Sue39, it's not possible for a guest to pay only a portion and the rest on arrival. If he has actually booked via AirBnB - and has comfirmation of the booking - then he has already paid the full fee. Have you checked to make sure that the lesser fee does not include the parts of a fee that AirBnB deducts as part of its service?
Has he in fact booked via AirBnB, or has he contacted you outside of the website? If so, this is against regulations.
I'd suggest that you take time to read the How To Guidelines carefully. They'll inform you well, and help keep the stress level down. 🙂 You'll find a number of them by typing keywords or questions into the 'search the community' box at the top of the page.