Is it permissible to waive a grandmother's right to the shop, and what is the ruling on burning the document that proves her right in exchange for benefiting from it?

Old lease contracts are Islamically invalid because they do not specify the duration of the lease, which is a fundamental condition for the validity of the contract. If the duration is not specified, either party has the right to terminate the contract at any time. The law that permits the lease to remain for the tenant's lifetime and be inherited, while the rent remains fixed, is invalid and contrary to Islamic law. The tenant is considered an usurper (ghasib) if he insists on retaining the leased property based on this law, for indeed, "the law does not make forbidden what is lawful nor lawful what is forbidden." Any lease contract that does not specify a duration must be terminated, the leased property returned to the owner, and fair market rent paid for the period of use.