Is investing an amount of 100 million dinars with a person who works in the field of Forex trading, on the condition that the investor receives 20% monthly of the capital, with a guarantee of the capital after two years, permissible (halal)? And what is necessary to correct the contract for it to become a legitimate Mudarabah (profit-sharing partnership)?

The validity of mudarabah (profit-sharing partnership) requires agreement on a common, undivided share of the profit, and that any loss be borne by the capital, while the mudarib (investor) only loses their effort, unless there was transgression or negligence on their part. As for investing in the currency market (Forex), its permissibility depends on observing Shariah controls, such as ensuring qabdh (possession/delivery) and avoiding impermissible elements like leveraged trading, overnight fees, selling what one does not own, and trading in prohibited assets.