create table customers(
  cust_id number,
  balance number(15,2)  --- для должника balance < 0
);

create table calcs(
  month_calc date,
  cust_id number,
  calc_money number(15,2),  --- всегда положительное число
  pay_money number(15,2)   --- всегда положительное число
);

create global temporary table tmp_customer_debts(
    cust_id number,
    balance number (15,2) default 0,
    cnt_month smallint default 0,
    constraint PK_CUST_ID primary key (cust_id)
) on commit preserve rows;

insert into customers values(1,-200);
insert into customers values(2,-250);
insert into customers values(3,-300);
insert into customers values(4,-100);

declare
    i smallint:= 0;
    dd date:= to_date('2016.12', 'yyyy.mm'); 
    
begin
    loop
        insert into calcs values(add_months(dd,-i),1,50,0);
        insert into calcs values(add_months(dd,-i),2,50,0);
        insert into calcs values(add_months(dd,-i),3,50,0);
        insert into calcs values(add_months(dd,-i),4,50,0);
        
        i:= i + 1;
    exit when i > 10;
    end loop;
    
end;

declare
    s_month date := to_date('2016.12', 'yyyy.mm');
    i smallint := 0;
    c_balance number(15,2);
    c_money number(15,2); 
    
begin
    --очистить вр.таблицу
    execute immediate('truncate table tmp_customer_debts');
    
    --выбрать должников
    insert into tmp_customer_debts(cust_id, balance)
    select cust_id, balance
    from customers
    where balance <= -0.01;
        
    -- цикл по должникам
    for rec in ( select cust_id, balance from tmp_customer_debts ) loop
        i:= 0;
        
        c_balance:= rec.balance;
        c_money:= 0;
            
        loop
            select calc_money into c_money
            from calcs
            where month_calc = add_months(s_month, -i) and cust_id = rec.cust_id;
            
            --dbms_output.put_line('cust_id: '||rec.cust_id || '  c_money:' || c_money || '  c_balance:' || c_balance || '  i:' || i);
                
            c_balance:= c_balance + c_money;
            
            i:= i + 1;
        exit when (i = 10 or c_balance >= 0);
        end loop;
            
        update tmp_customer_debts
        set cnt_month = i
        where cust_id = rec.cust_id;        
    end loop;
end;    

select *
from tmp_customer_debts

select
    cust_id,
    balance,
    count(1) as cnt_month
from
    (
        select c.balance, s.cust_id, sum( calc_money) over (partition by s.cust_id order by month_calc DESC) as s_sum
        from
            sam_tmp_calcs s
            inner join sam_tmp_customers c on c.cust_id = s.cust_id
    )
where
    balance + s_sum <= 0 
group by
    cust_id, balance
order by 1