Summing different tables values limiting the results
You might try this:

select
  sum(amount) amount
from(
  select
    least(sum(amount),10000) amount
  from (
    select date,
           200 amount
    from   a
    union all
    select date,
           500 amount
    from   b) list_of_all
  group by
    date(date)) list_by_day