Is it permissible to annul a lease contract after five years out of an original fifteen, and reclaim the remaining rental amount, or bring a substitute tenant to cover the remaining amount?

Your question involves two issues:

First: Terminating the rental agreement between you and the landlord. There are two scenarios for this:

1. If the landlord does not agree: You are obligated to fulfill the contract, based on the verse: (Fulfill [your] contracts). It is commendable for the landlord to accept the termination, based on the Prophet's (PBUH) saying: "Whoever rescinds a contract for a Muslim, Allah will forgive his slip."

2. If the landlord agrees: There is no objection to this, and he will return to you the amount corresponding to the remaining period. This transaction is considered an "iqalah" (mutual rescission). If the rescission is for more or less than the original price, the majority of scholars hold that it is a termination of the previous contract and is not permissible. However, Abu Yusuf and Muhammad ibn al-Hasan (definition) consider it a new contract and deem it permissible.

Second: Your subletting the apartment to another person (subleasing the rented property). This is permissible according to the majority of scholars (the Shafi'is, the Hanbalis, and the jurists of independent reasoning), and we are not aware of any disagreement on this matter. So, you may sublet the apartment to whomever you wish during the rental period, whether you receive the rent directly or through the landlord.