Is there a way to do a partial write off on a invoice?

Answer: Yes.  First apply all the payments to the invoice and then click on the Write Off link.  Meaning the write off should be done after there are going to be no further payments to the invoice.  Write Off link will record a write off of the remaining due on the invoice.

Question: What if we are trying to write something off (like a discount amount) before we send the rest of the payment to client?

Answer: That would be a discount and not a write off.  To enter a discount in an invoice, please click on the Invoices tab of the case. Then click on the first icon in the invoice line (icon with a computer screen).  A popup will open. In the popup, scroll down towards the bottom and enter a discount amount in the discount box and then click save next to the discount box.  Close the popup and download PDF invoice again.