Here is the logic I use to add a new detail record to the grid:

LOCAL LN_ADDFLAG
LN_ADDFLAG = .T.

SELECT TRANFILE
SET FILTER TO
SET FILTER TO TRANFILE.N_TRANNO = RECEIPT.N_TICKETNO
*SET ORDER TO N_TRANNO
*SET FILTER TO TRANFILE.N_TRANNO = M.nCURR_TICKETNO
GO BOTTOM
IF EMPTY(TRANFILE.C_COMNO) AND !EMPTY(TRANFILE.N_TRANNO)
GO TOP
IF !EMPTY(TRANFILE.C_COMNO)
=MESSAGEBOX("THERE IS A BLANK TRANSACTION AT THE BOTTOM!"+CHR(13)+CHR(13)+"ENTER TRANSACTION THERE.",48,"")
ENDIF
LN_ADDFLAG = .F.
ENDIF
IF LN_ADDFLAG = .T.
*SELECT TRANFILE
APPEND BLANK
REPLACE TRANFILE.N_TRANNO WITH RECEIPT.N_TICKETNO
*REPLACE TRANFILE.N_TRANNO WITH M.nCURR_TICKETNO &&- connect transaction record with receipt
&&- record via. receipt number
*SET FILTER TO TRANFILE.N_TRANNO = M.nCURR_TICKETNO
SET FILTER TO TRANFILE.N_TRANNO = RECEIPT.N_TICKETNO
GO BOTTOM
THISFORM.PAGEFRAME1.PAGE1.GRID1.COLUMN1.CMBO_COMNO.SETFOCUS()
THISFORM.PAGEFRAME1.PAGE1.GRID1.REFRESH
ELSE
GO BOTTOM
THISFORM.PAGEFRAME1.PAGE1.GRID1.COLUMN1.CMBO_COMNO.SETFOCUS()
THISFORM.PAGEFRAME1.PAGE1.GRID1.ACTIVATECELL(RECNO(),1)
THISFORM.PAGEFRAME1.PAGE1.GRID1.REFRESH
ENDIF