You can enter refunds made outside of billwerk for a respective contracts as follows:
1) Go to the contract details of the customer concerned and click on "Perform action > capture ext. Payment / Refund".
2) In the opened dialog select the "Refund" tab.
3) Enter the posting text as description (also displayed as such in the financial accounting export). If the refunded amount exceeds the account balance, the payment of the excess amount is immediately triggered when recurring payment is activated. You can deactivate the recurring payment before you confirm the refund by checking the box below the amount. In the preview you can see the old and the new account balance.
4) Press "OK" to save the refund.