Is the tenant obligated to pay the full amount of rent for the property despite vacating it, and the landlord leasing it to another tenant three months before the end of the first contract?

The lessor is not permitted to lease out the rented property for the remaining contract period unless the lessee waives their ownership of the house's usufruct for that period. If the lessee waives it, then they are obligated to pay the rent. However, if they do not waive it, the contract is void, based on the Prophet's (peace and blessings be upon him) saying to Hakīm ibn Hizām: "Do not sell what you do not possess." The owner or friend must pay the fair market rent to the lessee for that period, because they colluded to usurp his right. The owner cannot compel the lessee to pay rent while also leasing the dwelling to another, as this constitutes consuming wealth unlawfully.