[bucanero]

prueba con esta consulta:

SELECT v.Mes, sum(v.Total) AS Total, sum(d.cantidad) AS Cantidad
  FROM VentasABC v
       LEFT JOIN (SELECT ID_Venta, sum(cantidad) AS cantidad
                    FROM DetalleABC
                   WHERE ID_Concepto IN (101, 102)) d
          ON d.ID_Venta = v.ID_Venta
GROUP BY v.mes