WC_Payment_Gateway::process_refund PHP Method

If the gateway declares 'refunds' support, this will allow it to refund. a passed in amount.

public process_refund ( integer $order_id, float $amount = null, string $reason = '' ) : boolean
$order_id	integer
$amount	float
$reason	string
return	boolean	True or false based on success, or a WP_Error object.

    public function process_refund($order_id, $amount = null, $reason = '')
    {
        return false;
    }

WC_Payment_Gateway