You can lock Invoices to help prevent any additional changes from being made to an Invoice. Locking an Invoice essentially disables the Save buttons at the bottom of the Invoice. Changes can still be made after the Invoice is unlocked.
Invoices are automatically locked when they are emailed or printed. This is done because the system assumes that the Invoice has been sent to the Client therefore additional changes should not be made.
You can also lock Invoices when the print or email flags are removed. You can clear flags by selecting "Clear Flags" from the Actions menu:
This will open the Clear Print/Email Flags dialog:
When you check the box for either the email or print flags, you will also be able to check the box next to Lock invoices. Click Clear Flags to complete.
You can also lock Invoices on an Invoice by Invoice basis. From the Invoice overlay, check the box next to Locked and click Save to lock the Invoice.