SELECT 
    t1.fecha,
    t1.total - COALESCE(t2.total, 0) AS Total
FROM 
    (SELECT DATE_FORMAT(fecha, "%Y-%m-%d") AS fecha, SUM(app_ventas.total) AS total
     FROM app_ventas
     WHERE fecha >= ""  AND    fecha <= ""  AND app_ventas.estado = 1 
     GROUP BY DATE_FORMAT(fecha, "%Y-%m-%d")) t1
LEFT JOIN 
    (SELECT DATE_FORMAT(fecha, "%Y-%m-%d") AS fecha, SUM(valor_egreso) AS total
     FROM app_egresos
     WHERE fecha >= ""  AND    fecha <= ""
     GROUP BY DATE_FORMAT(fecha, "%Y-%m-%d")) t2
ON t1.fecha = t2.fecha;