When setting up an e-conomic accounts in Fenerum one must choose "Debtor settlement account". This article described the purpose of this account.
In case of settlements (using credit note to pay for other invoice) Fenerum will book two payments - one from "credit note" to special "Debtor settlement account" and second from this account to the invoice paid by settlement.
Settlements of the invoice will also happen if you are refunding (canceling) not yet paid invoice.
Fenerum will then create a credit note to zero the balance of the not paid invoice.
The balance on this special account should always be 0 as Fenerum will always create two transactions there.
It's possible that you will not have this special account on your account plan. In this case simply create a new account with type "Balance sheet".