What is the solution to the problem of a friend working in China, where the factory owner agreed with him on a salary of 2000 dollars per month, then he discovered that the salary is 800 dollars, and that 2000 Chinese Yuan is equivalent to 300 dollars, and the factory owner denies the agreement?

If the lessor and the lessee dispute the amount of the rent, and neither has proof, then they must both take an oath (yataḥālafān). If they both swear, the lease is annulled, and the fair market rent (ajrat al-mithl) becomes obligatory, as estimated by experts. If one of them refuses to take the oath, then judgment is rendered in favor of the other party.