In the listing of Invoices under "Clients" or "Cases (Projects)" tabs, "Write Off" links are displayed below the invoiced amounts for each invoice in the list.  Clicking on this link will cause the remaining due to be written off.  For written off invoices, a link to revert the write off will appear.  

After an invoice is written off, the invoice will not be displayed on the dashboard and the remaining amount will not be considered (as previous due) in subsequent invoices in the same case.