What is the ruling of Islamic law regarding the insistence of the younger sister's husband on implementing the original agreement to pay 20,0 pounds annually, and his inability to secure a larger amount? And does his share of the building equal half, or is it proportionate to his contribution? And what is the ruling of Islamic law regarding the elder sister's husband's request to reclaim the debt in a lump sum or through the four proposed alternatives, given his changed financial circumstances?

The younger sister's husband is not obligated by the proposals presented to him by the older sister's husband. Both parties must adhere to the agreed-upon division of the building and the installment payment plan, unless they mutually agree otherwise. Scholars have differed regarding the ruling on deferring loan repayment. The majority (Hanafis, Shafi'is, and Hanbalis) hold that deferring loan repayment is impermissible and is not binding if stipulated in the contract. However, the Malikis permit deferring loan repayment, considering it binding if specified at the time of the contract. This view was favored by Sheikh al-Islam Ibn Taymiyyah and deemed correct by al-Mardawi.