with credits as(
select 1 user_id, 1 doc_id, {d '2020-01-01'} doc_date, 100 amount union all
select 1 user_id, 2 doc_id, {d '2020-01-03'} doc_date, 200 amount union all
select 1 user_id, 3 doc_id, {d '2020-01-05'} doc_date, 300 amount union all
select 2 user_id, 4 doc_id, {d '2020-01-11'} doc_date, 400 amount union all
select 2 user_id, 5 doc_id, {d '2020-01-21'} doc_date, 500 amount
),
payments as (
select 1 user_id, 250 payment, {d '2020-01-21'} payment_date union all
select 1 user_id,  50 payment, {d '2020-01-21'} payment_date union all
select 1 user_id, 250 payment, {d '2020-01-21'} payment_date union all
select 2 user_id,   5 payment, {d '2020-01-21'} payment_date
)
select cr.*,
       sum(amount) over(partition by cr.user_id  order by cr.doc_date) amount_inc,
	   case
	        when sum(amount) over(partition by cr.user_id  order by cr.doc_date) <= p.payment then 0
	        when -amount+sum(amount) over(partition by cr.user_id  order by cr.doc_date) < p.payment then sum(amount) over(partition by cr.user_id  order by cr.doc_date)-p.payment
	        else amount
	   end
  from credits cr
  join
  (
  select user_id, sum(payment) payment
    from payments
   group by user_id
  ) p
  on p.user_id = cr.user_id