The most efficient way to write off an unpaid invoice amount is by using a separate payment method and registering it as a normal payment.
To start, add a new payment method in Settings, Account receivables, Payment methods
In field Financial account, select the G/L account you want to use for the write off.
To execute the write off, go to the relevant invoice:
In this example, we have an invoice with total amount 6107,50 where 6107 has been paid and we want to write off the last 0,5.
Open the invoice by clicking the invoice number.
Select Register payment in the invoice header context menu:
Add a payment of 0,5 with the payment method for write offs:
Invoice is now marked as Settled and Payment postings will be as following: