You can make changes to line items on the invoice and they will not impact the billed amount on the Billable Entries tab.
You will have the option for additional Detailed Description rows when you click Add to Invoice. This allows you to enter a description and amount directly on the invoice that will not appear as an adjustment on the Billable Entries tab. You will still have the options to add Time & Expenses or a flat amount to the invoice.
The total of all invoice line items must match the total billed amount.