If Manager would be doing something that would reassemble spyware, we would be already blacklisted by every single antivirus software.
The only weak spot I’m aware of could be with “Email” button. When you send emails directly from Manager, the email will travel to recipient through our servers. If you don’t like that, I recommend don’t use “Email” button and simply print to PDF and send the email from your preferred email client. You will be trading convenience for extra security but I don’t think it makes much sense as emails are insecure by design and always have been.
Manager never calls “home” to tell us names of your businesses, customers, suppliers or any information which you enter into Manager - not even accidentally. For example, if your software crashes, the bug report could contain some of these information accidentally and for this very reason Manager doesn’t send us bug reports automatically. Instead you can review, sanitize and manually send the bug report after you are satisfied with what you are disclosing or choose not send us anything at all.
You can verify that Manager doesn’t depend on Internet connection by simply disconnecting from the Internet. Manager will be working exactly the same (except for the email function, you can’t send emails when offline).