A fully paid refund is simply recorded as a negative entry on the Income Account and a negative payment on the bank clearing account used:
$100 refund
$100 credit from the Income account representing the refund and paid from the Cash account.
A partially paid refund is handled as a reverse of a LayBy thus it will create a negative entry on the LayBy account.
$1,100.00 Refund paid only $400.00 back to the customer will generate:
-$400.00 LayBy transaction with a payment of $400.00.
There is an implicit assumption that the refund will be fully paid at a later date.