Select Periodo, Libro, Voucher,
SUM (iif (MOV.DH = 'D', MOV.n_IMP, 0.00)) AS DEBE,
SUM (Iif (MOV.DH = 'H', MOV.n_IMP, 0.00)) AS HABER ,
SUM (iif (MOV.DH = 'D', MOV.n_IMP, 0.00)) - SUM (Iif (MOV.DH = 'H', MOV.n_IMP, 0.00)) AS Dif
from tblMovimientoContable MOV
group by Periodo, Libro, Voucher
having SUM (iif (MOV.DH = 'D', MOV.n_IMP, 0.00)) - SUM (Iif (MOV.DH = 'H', MOV.n_IMP, 0.00)) > 0