Did the number values come through correctly with the debug formulas above? If so, then let's switch things around a bit. If you just need a date-like display and don't need an actual date data type, you could do something like this:
mid({Sales.InvoiceNumber},3,2) + "/" + mid({Sales.InvoiceNumber},5,2) + "/" + ToText(Year({Sales.InvoiceDate}))
-Dell