How can I correct a transaction that has mistakenly appeared in previous (closed off) financial year?

Hi Margriet 

Thank you for the screenshots. 

When you recorded the initial deposit/payment to the order, it would have been received into a bank account, doing the refund now would cancel that effect. You can look for the initial deposit by clicking into the invoice > click on History next to the amount applied > look for the payment received line (transaction type CR), click on the arrow next to it to bring up the Receive Payment window, it will show the bank account it was deposited into.

Your screenshots shows that the payment received on the order was not the full amount. So with the credit note created from the reserved invoice, please first apply the credit note to the original invoice to close it off. The remaining credit should be the same amount of the original deposit, refunding that would return the bank balance back to before the deposit was first recorded. 

If you have any further questions please feel free to post again.