Is it permissible, Islamically, for a brother to benefit from his elder brother's apartment (the more spacious and expensive) for three years, and then for the elder brother to benefit from his younger brother's apartment (the more cramped and cheaper) for nine years? And does this agreement fall under the ruling of "lend me and I will lend you" or "give me a loan and I will give you a loan," which is prohibited?

It is permissible to lease a usufruct for a usufruct of the same kind, such as accommodation for accommodation, according to the majority of scholars, in contrast to the Hanafis. Usufructs, in their view, are like existing tangible assets, and usury does not apply to them except if its cause is present, which is absent here. As for what was mentioned in the question about the tenant benefiting from his brother's apartment "at a later time," this is incorrect because it renders the lease invalid due to the unspecified duration of the lease, including its start and end dates. However, it is not a condition that the benefit begins immediately after the agreement; rather, the start and end dates of theufruct must be known and agreed upon.