Is it permissible for a subcontractor to pay another subcontractor part of the value of work the latter has completed to expedite receiving his money, in exchange for the first subcontractor receiving the full value from the main company after a month and benefiting from the difference?

The transaction mentioned, which is the contractor selling his debt to the company for accelerated cash payment, is a form of forbidden debt sale, because it leads to usury (riba). It is not permissible to sell a deferred debt to anyone other than the debtor for an accelerated or deferred cash payment, whether it be of the same type or a different type, because that involves selling a debt for a debt (بيع الكالئ بالكالئ), which is prohibited.