Related Questions:
What happens to an invoice once its been sent?
Can an invoice be edited after its been sent to the client
Answer:
Connected Invoices
Connected Invoices in Event Temple are designed to update automatically as changes are made to the Booking so that manual updates are not necessary. As such, there is no ability currently to lock Connected Invoices. These will be phased out in the near future if you use an 'Invoice' you now have the ability to Finalise (lock) the Invoice. (see below)
Invoices
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.