Is it permissible to sell services and receive their price before they are rendered, knowing that I do not currently own them? And if I am not the one who renders the service, but rather request it from another person and then sell it to my client for a profit, is that permissible?

A contract for usufructs is a lease (ijara), and so are services. The usufructs are not in existence at the time of the contract but come into existence later. Ibn al-Rif'ah and Ibn al-Qayyim stated that leasing non-existent usufructs and forward selling (bay' al-salam) are permissible by analogy, because it is a guaranteed sale that is a debt (in dhimma), described and deliverable. It is permissible for the hired person (ajir) to hire another to perform the work, unless it was stipulated that he must perform the work himself. If the raw materials are provided by the worker, then the contract is one of istisna' (manufacturing contract), combining elements of lease and sale. In such a contract, the price may be paid in advance or deferred, and it is not a condition that the work be done by the worker himself unless stipulated.