-- CREATE TEST DATA
create table #tmp (id int, amt int);
insert into #tmp values (1,5000),(2,3000),(3,4000),(4,8000),(5,6000);

-- SET THE CUT-OFF VALUE AS A FRACTION.
declare @cutoff decimal(9,4);
set @cutoff = 0.80;

-- THE QUERY
with cte1 as
(
   select *,row_number() over (order by amt) as RN,
   (1.0-@cutoff)*sum(amt) over() as Cutoff
   from #tmp
),
cte2 as
(
   select *,amt as RTotal  from cte1 where RN=1
   union all
   select c1.*,RTotal+c1.amt
   from cte1 c1 inner join cte2 c2 on c2.RN+1 = c1.RN and c2.RTotal+c1.amt  <= c2.Cutoff
)
--select sum(amt) from cte1 where RN > (select max(RN) from cte2);
select * from cte1 where RN > (select max(RN) from cte2);

-- CLEANUP.
drop table #tmp;