Can I have Project Managers approve invoices before they are sent out?
Yes, you can set Project Managers to approve invoices.
An example process could be for Admins to create the invoice, and the Project Managers be set as "Billing Managers" on the project. Once Admins have created the invoice in the Draft status, the Project Managers can then approve the invoice by clicking the Complete button. Once the Admin sees that the invoice is in the Completed status, the Admin can then send the invoice to the client.
Note that by default in Unanet, time/expense must be approved prior to invoicing.