You can edit a case invoice once it is created and in Draft status by clicking on the View link:
On the right hand side of the new window, you can enter an Adjustment type and the Adjustment amount (only numbers, no symbols), and Preview to see what the invoice looks like with these changes:
From this screen you can also export the invoice to PDF, Excel, etc.:
When the invoice is exported to Excel, it can be further edited by adding/removing rows.
If the adjustment amount entered is greater than the total amount of the invoice, the balance will not show a credit; it will show $0.00:
The edits made to an invoice cannot be saved over the invoice created in Gardiant. Please keep track of these changes in your accounting system for accuracy purposes.