SQL oracle пересчет сумм в неотрицательные

Сначала вычисляете нарастающий итог, потом ищете в нём "будущий минимум":

Подготовка таблицы

create table sums (contract_id number, dt date, amount number);

insert into sums (contract_id, dt, amount)
    select 1 as contract_id, date'2024-01-01' as dt,  100 as amount from dual union all
    select 1 as contract_id, date'2024-01-02' as dt,  500 as amount from dual union all
    select 1 as contract_id, date'2024-01-03' as dt,  300 as amount from dual union all
    select 1 as contract_id, date'2024-01-04' as dt, -700 as amount from dual union all
    select 1 as contract_id, date'2024-01-05' as dt,  -50 as amount from dual union all
    select 1 as contract_id, date'2024-01-06' as dt,   50 as amount from dual union all
    select 1 as contract_id, date'2024-01-07' as dt,  -30 as amount from dual union all
    select 1 as contract_id, date'2024-01-08' as dt,  100 as amount from dual;

Запрос

select
    contract_id,
    dt,
    amount,
    greatest(0, min(acc) over (partition by contract_id order by dt desc) - acc + amount)
        as positive_amount
from
    (
        select
            contract_id,
            dt,
            amount,
            sum(amount) over (partition by contract_id order by dt) as acc
        from
            sums
    )
order by
    contract_id,
    dt