Is it possible to use LAG() with WHERE?
Let me know if this solve your problem.

I've set up a rextester example to test it. 

I've used this data:

create table #mov(produto_id int, documento int, data_cadastro datetime, entradas int, salidas int);

insert into #mov values
(1, 1, '2017-01-01', 10, 0),
(1, 1, '2017-01-02', 10, 2),
(2, 1, '2017-01-01', 10, 0),
(1, 1, '2017-01-03', 10, 0),
(3, 1, '2017-01-02', 10, 0);


To get the cumulative sum, apply SUM() OVER clause in this way:

select 
    produto_id, 
    documento,
    entradas,
    salidas,
    sum(entradas-salidas) over (partition by produto_id order by produto_id, data_cadastro
    rows between unbounded preceding and current row) as acumulato
from
    #mov


This is the result:

+------------+-----------+----------+---------+-----------+
| produto_id | documento | entradas | salidas | acumulato |
+------------+-----------+----------+---------+-----------+
|      1     |     1     |    10    |    0    |     10    |
+------------+-----------+----------+---------+-----------+
|      1     |     1     |     0    |    2    |     8     |
+------------+-----------+----------+---------+-----------+
|      1     |     1     |    10    |    0    |     18    |
+------------+-----------+----------+---------+-----------+
|      2     |     1     |    12    |    0    |     12    |
+------------+-----------+----------+---------+-----------+
|      3     |     1     |     5    |    0    |     5     |
+------------+-----------+----------+---------+-----------+


If you need the last acumulato in the same row, you can use:

;with cteSum as
(
    select 
        produto_id,
        data_cadastro,
        documento,
        entradas,
        salidas,
        sum(entradas-salidas) over (partition by produto_id order by produto_id, data_cadastro
        rows between unbounded preceding and current row) as acumulato
    from
        #mov
)
select produto_id, documento, entradas, salidas, acumulado, 
       lag(acumulato,1,0) over (partition by produto_id order by produto_id, data_cadastro) as last_acm
from cteSum;

+------------+-----------+----------+---------+-----------+----------+
| produto_id | documento | entradas | salidas | acumulato | last_acm |
+------------+-----------+----------+---------+-----------+----------+
|      1     |     1     |    10    |    0    |     10    |     0    |
+------------+-----------+----------+---------+-----------+----------+
|      1     |     1     |     0    |    2    |     8     |    10    |
+------------+-----------+----------+---------+-----------+----------+
|      1     |     1     |    10    |    0    |     18    |     8    |
+------------+-----------+----------+---------+-----------+----------+
|      2     |     1     |    12    |    0    |     12    |     0    |
+------------+-----------+----------+---------+-----------+----------+
|      3     |     1     |     5    |    0    |     5     |     0    |
+------------+-----------+----------+---------+-----------+----------+