On the import, field settings you would check the show column box for Invoice # to import the data for that field. If the data in the invoice column is blank, Sage will import a blank invoice number as long as you have that same column show box checked when you export data.   However if you import the same data again, Sage will duplicate all the records for the invoice numbers that are blank.  If the invoice number is not blank, sage will not allow a duplicate record to import.