Does my brother have a right to the difference in the house's price if I sell it now, knowing that I bought his share from him on condition that I give him the price difference if I sell the house within 5 years, and nearly four and a half years have passed, and the price per square meter in the house has risen from 700-1500 EGP at the time of purchase to approximately 5000 EGP now, and I had previously paid him an additional amount to raise the price per meter at which he sold his share?

This issue pertains to the combination of a sale and a condition within a single contract, a matter of dispute among jurists. The majority of them invalidate both the condition and the contract, while the Hanbalis invalidate the condition but validate the contract.

The condition mentioned in the question renders the price unknown, which, according to Al-Khattabi, invalidates the sale. According to the Hanbalis, if the non-fulfillment of a condition leads to the frustration of a purpose for the one who stipulated it, then the madhhab (school of thought) is that he has the choice either to annul the contract or to take compensation for the decrease in price due to the annulment of the condition.

Based on the question, it appears that the brother did not agree to the price except on the condition of receiving the price difference later. If we rule that the sale is valid and the condition is void, then the seller has the right to demand compensation for the decrease in price. However, if we rule that the sale is void, then the price returns to the buyer and the sold item returns to the seller, which harms the buyer. Therefore, it is preferable for the two parties to reconcile or to resort to a Sharia judge or an arbitration council.