It's easier now:
In calling form You must use NAME to invoce the sum method:

something like that:

&& In calling form
DO FORM FrInvoice NAME thisform.frinvoice .....
&& In method whereYou need the SUM()
IF VARTYPE(thisform.frinvoice) == "O" AND .NOT. ISNULL(thisform.frinvoice)
   thisform.frinvoice.Sum()
ELSE
   DO FORM FrInvoice NAME thisform.frinvoice ..... NOSHOW
   thisform.frinvoice.Sum()
ENDIF