create table #t
(
  充值 int,
  已退 int,
  可退 int
)
insert into #t(充值, 已退, 可退)
values (200, 100, 100), (500, 200, 300), (300, 100, 200)

/*
作者：zhang502219048
腳本來源：https://www.cnblogs.com/zhang502219048/p/14127208.html
*/

declare @i要退 int = 450;
with cte1 as
(
  select *, row_number() over(order by 可退 desc) rn, 0 可發起退款, 0 待退
  from #t
),
cte2 as
(
  select rn, 充值, 已退, 可退, 
    可發起退款 = case when @i要退 > 可退 then 可退 else @i要退 end, 
    待退 = @i要退 - case when @i要退 > 可退 then 可退 else @i要退 end -- 待退 = 要退 - 可發起退款
  from cte1
  where rn = 1
  union all
  select t2.rn, t2.充值, t2.已退, t2.可退,
    可發起退款 = case when t1.待退 > t2.可退 then t2.可退 else t1.待退 end, 
    待退 = t1.待退 - case when t1.待退 > t2.可退 then t2.可退 else t1.待退 end
  from cte1 t2
  inner join cte2 t1 on t1.rn = t2.rn - 1 -- t2是t1的下一條記錄
  --where t2.rn > 1 and t1.待退 > 0
)
select * from cte2

drop table #t