@Tom385 You only need to refund that amount specified in your cancellation policy. AirBnb arrange refunds and it is them who issue refunds. Don’t pay the payment request.
Did your guest cancel or just not turn up?
If they didn’t cancel they would be in a position of needing to pay the whole booking up until their planned departure or until they cancel.
The guest sending you a payment request for a refund could be construed as an attempt at extortion in exchange for a favourable review. I would contact CS to inform them of the payment request and that you feel you are being held to ransom to pay monies in exchange for a favourable review. The AirBnb review process forces this fear on hosts and it should just not be tolerated.