What is the extent of permissibility, Islamically, of concluding partnership contracts between spouses regarding their properties, both the standard and the expanded forms, and is the expanded contract considered an injustice to the heirs' rights? And what is the fundamental principle in Islamic law concerning this?

The basic principle is that an adult, sane, and mature individual has the legal capacity to manage their own wealth. If a husband and wife voluntarily agree to share their present assets, there is no objection to that. However, regarding sharing future assets, the majority of jurists consider it invalid because a gift is not permissible for something non-existent or unknown. This system may lead to an infringement on the rights of heirs in the event of death, or the rights of one of the spouses in the event of divorce. Furthermore, the origin of this law is imported from Christians and is based on a principle that is invalid in Islamic law, namely, the eternity of conjugal life. Therefore, it must be rejected as long as it is optional.