Is profiting from supplying experts and technicians to international companies permissible, where lower salaries are paid to employees based on a prior agreement?

As long as the conditions for the lease are met and there are no impediments, there is no objection to sub-leasing the leased item, whether the leased item is a person, an animal, a car, or a house. This is because the lessee owns the usufruct through a valid contract, so he may lease it to another like himself for more or less than the rent, or he may donate it.