Is it permissible to "buy a key"—which is paying a large sum to the owner for accommodation at a reduced rent, such that the buyer gains the right to dispose of the house and rent it out to others in exchange for recovering the amount paid—in order to lower the cost of rent, and if not, is there a permissible way to achieve the same purpose?

It is permissible for the tenant to lease the property for a rent less than or greater than the rent he contracted for with the owner. He may also take in return for that what is known as "key money" (shira' al-miftah) or "khulu al-rijl" (premium for vacating), unless the contract with the owner stipulates otherwise. However, if the lease period has expired, he is not permitted to lease the property except with the owner's consent; otherwise, what he takes as rent would be unlawful. If the tenant has possessions in the premises (such as furnishings or decorations), there is no objection to selling them to the owner or the new tenant for a known price.