Does the tenant's dwelling with his family in an old-lease apartment for less than its due value constitute a sin? Is it permissible to vacate this apartment in exchange for a sum of money from the owner equivalent to the cost of finishes and repairs spent on it over thirty years? And is there usury in that?

The old tenancy contract is Islamically void (batil (definition)). The tenant must terminate it and return the property to its owner, or conclude a new, Sharia (definition)-compliant contract. The tenant must also pay the fair market rent for the previous period, unless the owner waives it. The tenant is considered a usurper (ghasib) if he does not respond to the owner's request to hand over the property. It is not permissible to benefit from the property without the explicit consent of the owner. It is not permissible for the father to take money from the owner in exchange for vacating the house, as this is consuming people's wealth unlawfully (akl amwal al-nas bil-batil), based on the Almighty's saying: "O you who have believed, do not consume one another's properties unlawfully except by trade [involving mutual consent] from among you." (Quran 4:29). Repairs are considered actions of a usurper if done without the owner's consent. If done with his consent, they are deducted from the rent and their value at the time of expenditure is reimbursed.