Is it permissible to increase the second wife's allowance, or give her gifts, or arrange trips for her, or buy gold for her, as compensation for her humbler dwelling? Is it permissible to allocate a portion of the rent from a second apartment for her, given the inability to achieve complete justice and the heart's inclination towards the second wife? What is the general advice for this problem?

Justice between wives is divided into what is obligatory and what is not obligatory. The obligatory aspects include maintenance (nafaqah), clothing (kiswah), lodging (mabit), and housing (sakan). This is an achievable matter, and whoever cannot fulfill it is forbidden from polygyny, as Allah says: (But if you fear that you will not be able to do justice, then [marry only] one) [An-Nisa: 3]. As for the non-obligatory justice, it is justice in heartfelt affection, which is not achievable, as Allah says: (And you will never be able to do justice between wives, even if you should strive to do so. So do not incline completely [toward one]) [An-Nisa: 129]. Therefore, it is not permissible to incline completely toward one of them. It is obligatory to fear Allah regarding the first wife and to achieve the obligatory justice with her, or to divorce her kindly, or to reconcile with her and for her to relinquish some of her rights. Likewise, it is not permissible to grant the second wife advantages over the first unless justice is achieved between them.