What is the legitimacy of the capital owners demanding a percentage of the profit (20,000) monthly, with the remainder being saved in a special account for them until they request it, knowing that the mudarib (working partner) is entitled to one-third of the profit?

As long as the contract with you is for one-third of the speculative profit, and you do not guarantee the principal or the profit, there is no objection to paying him a fixed monthly amount under the account of his profits. Then, at the end of the year, you calculate what he received and what he is entitled to from the profit. If what he received is more than his share, he returns the excess. If his share is more, he takes the remainder. For profit is due upon its emergence and owned upon liquidation or valuation, and amounts distributed before liquidation are considered paid on account.