What is the religious ruling regarding a project in which three brothers participated on land owned by one of them, the price of which was paid by the partners without documentation, and now the owners of the land are demanding the current price of the land, not the land itself, due to the lack of documents, while the partners want their share in the land, and what is the duty of the land owner's children if he passes away?

Allah Almighty commanded the documentation of contracts to preserve rights and resolve disputes. There is no difference in this between siblings and others; in fact, siblings are more deserving of it. This command is a guidance towards what is beneficial, not an obligation or compulsion.

If a partnership or sale contract is concluded without documentation, the contract is not void. If the claim is proven by the defendant's admission or by other evidence, its requirements are acted upon, and judgment is rendered in favor of the claimant.

Therefore, if someone claims to have paid the price for a part of a land, and this is proven by the admission of the owner or their heirs, or by evidence, the lack of written documentation does not invalidate the claim. Neither the heir nor anyone else has the right to prevent him from his share in the land, or to restrict him from it, or to pay the price for his share without his consent, if his purchase of that part from the original owner is proven.

The questioner is advised to present his problem to the Sharia (definition) courts.