When the email with the invoice is sent to the customer, it summarises the invoice number and the amount, however the amount does not have a $sign. Also when a part payment has been made and you resend the invoice to the customer, the remaining balance is not reflected in the email text. It still shows the full amount rather than the new amount owing after the part payment.
The team fixed the default email text to include a currency symbol a while back, however (technically) we could only address NEW books created after the update.
Existing books pre-update can change the default email text manually to add the $ symbol.