Conditionally sum a column based on a mod result
As my boss stated, all I had to do was edit the inner statements instead of trying to manipulate the results on the outer query. We managed to do what I needed with this:

SELECT  y.datamov, 
    y.n_conta,
    y.sinal,
    y.valor,
    a.nome,
    '',
    0,
    y.lrdarc,
    y.quantidade
FROM (
    SELECT k.datamov,
        k.n_conta,
        k.sinal,
        sum(k.qtde_valida) as quantidade,
        k.grupo as lrdarc,
        sum(k.total) AS valor
    FROM (
        SELECT DATE (b.dt_normal) AS datamov,
               b.historico,
               a.obs,
               a.total,
               b.sinal,
               c.nome,
               a.n_conta,
               c.lrdarc,
               case when mod(c.lrdarc, 2) !=0 then
                    a.qtde
               else
                    0
               end as qtde_valida,
               trunc(c.lrdarc / 10) as grupo
        FROM caixa.rtitens a
        LEFT JOIN caixa.rtlan b ON (b.numero = a.n_lan)
        LEFT JOIN caixa.rtctares c ON (c.numero = a.n_conta)
        WHERE DATE (b.dt_normal) BETWEEN DATE ('2010-02-13') AND DATE ('2010-02-20')
        AND c.lrdarc != 0 AND b.sinal = 'E'
        ORDER BY 1, 7, 5
        ) k
    GROUP BY 1, 2, 3, 5
    ORDER BY 1, 2, 3
    ) y
LEFT JOIN caixa.rtctares a ON (a.numero = y.n_conta)
ORDER BY 1, 2, 3


Note that the select from the most inner query, where we've added this:

...
c.lrdarc,
case when mod(c.lrdarc, 2) !=0 then
     a.qtde
else
     0
end as qtde_valida,
trunc(c.lrdarc / 10) as grupo


This solved the problem.