There is a function for writing off bad debts. This creates a credit note for the invoice you wish to write off and then automatically allocates the credit note to the invoice.
You can use this function as follows:
Select Debtors from the main menu
Then select Credit Control
Then select Write Off Bad Debts
Search for and select the invoices you wish to write off
Please note that only invoices will be displayed
Click Process
Click Yes when asked OK to process the selected lines?
Select the Analysis Code you wish to use
Click Save