vWork offers the ability to set invoicing permissions at either an account level or at a user level.
You can control mobile worker invoicing permission at an account level via Settings > Invoicing.
These settings allow you to fine tune what is adjustable on the invoice, description, quantity, tax rate, line items, etc...
These setting also allow you to force mobile workers to use pre-defined pricebooks.
Mobile Worker User Settings
You can also change individual User Invoice Permissions here Settings > User to "None" "Full" or "Read".
Any change to the user's invoice permission will overwrite anything granted at an account level.
In the above examples, Mobile Workers have been given permission to Add, edit and delete line items as well as edit the "actual" quantity. However as this user has had their invoice permissions set to Read they will not have ability to change the invoice in any manner although they can view it.