@Mary64, do not accept cash, as neither you nor the guest would be protected under AirBnB's terms of service.
Unfortunately the situation arose because the pricing was not up to date on the calendar. A host cannot change the price once a guest has booked, unless the guest agrees. In which case you would need to change the price on the reservation. A message would be sent to the guest, and he would either accept or decline.
I understand the guest being angry. If you don't feel comfortable with him, however, you could either cancel the booking yourself (and face penalties, including the date being blocked so no one else can book), or you could ask the guest to cancel, and you would offer to reimburse the service fee that he paid to AirBnB.
A third option might be to apologise to the guest for the inconvenience, indicate that the booking can go ahead as is, and view the whole episode as a learning experience. 🙂
At the top of the General Hosting section is a stickied thread with a comprehensive set of links covering all kinds of AirBnB-related information with which hosts are urged to become familiar.