mas o menos lo he solucionado asi...
Código SQL
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Select Left(M.CUENTA,2) As CNivel, M.CUENTA, M.Libro, M.Voucher, M.RGt, M.Documento, M.FechaDocumento, M.Anexo, M.CCosto,
M.Mnd, M.Glosa, P.Descripcion,
iif (M.DH = 'D', M.n_IMP , 0.00) AS DEBE,
iif (M.DH = 'H', M.n_IMP , 0.00) AS HABER
from (tblmovimientocontable M
Left Join tblPlanContable P On (P.Empresa = '001' and P.Periodo = '2009' and M.Cuenta = P.Cuenta))
where M.Empresa = '001' and M.Periodo = '200902' order by M.CUENTA, M.FechaDocumento
Union all
Select Left(S.CUENTA,2) As CNivel, S.CUENTA, '' as libro, '' as voucher, '' as rgt, '' as documento,
'' as fechadocumento, '' as anexo, '' as ccosto, '' as mnd,
'' as glosa, '' as descripcion, 0 as debe, 0 as haber
from tblcuentasaldo S
where (S.nd_ant00 + S.nd_ant01) - (S.nh_ant00 - S.nh_ant01) <> 0 and nv = '6'
and S.Cuenta not in (Select Cuenta from tblmovimientocontable where Periodo = '200902')
order by S.Cuenta
aunque creo que mejor usare having