Is a person who has rented out an apartment, which he himself is renting under a lease-to-own agreement from a bank, committing a sin, and what should he do in the case of sin?

An "ijarah muntahia bittawleek" (lease ending in ownership) contract is permissible if it adheres to Sharia guidelines. The basic principle is to assume the validity of a Muslim's transactions. It is not a condition for the validity of the lease that the lessor owns the asset; rather, it is sufficient that he owns the usufruct (manfa'ah) through a lease contract.