Hi Dennis,
The invoice does not get a valid Invoice number until it is booked using the CurrentInvoice_Book method which returns the Invoice object. This object has the InvoiceHandle property which I believe is the number you are looking for.
If you need to find the invoice before it is booked, use the InvoiceHandle property from the CurrentInvoice, it will most likely allow you to find the not-yet-booked invoice, but it will not be a valid invoice number you can use later on after it has been booked.
# Bo