Related Questions:
Is there a way to lock an Invoice?
How do I stop people from adjusting Invoices?
How do I finalize and Invoice?
Can I modify and invoice after it has been finalized/locked?
Why can't I see the Finalize button on Invoices?
Answer:
Once an Invoice has been sent to a client you are able to Finalize the Invoice. This will prevent any changes from being made. You can Finalize using the button in the top right of the Invoice Screen. (This will not work for Connected Invoices)
The Finalise button will not be visible until the Invoice has been shared, or marked as sent using the Actions Button in the top right.
Once an Invoice is finalized you can no longer add or modify Items. You will only be able to Void or Delete the Invoice.