If the full refund as made through a case then you cannot refund any more through eBay.
You can make the payment to the buyer via a cheque, bank transfer or if you have a PayPal account a Friends and Family Payment.
If PayPal is used, email addresses need to be exchanged, this can sometimes be found with the order details >> Show Contact Info. If not and you need to ask the seller for this, make it very, very clear in your eBay messages, as eBay do read these, the reason for these exchanges. Make it clear it's not to be used for an off eBay sale.
@tinacd18