Is it permissible for brothers to demand money earned by their brother from cultivating agricultural land (including fallow land, orchards, and arable land) after they appointed him as their agent over it?

If the land is owned by you and your elder brother is appointed as an agent to manage it, then its benefits are for you, not for him. He must adhere to the agency agreement and work for your benefit. He is not permitted to exploit your property for his personal gain, and he has no right to take anything from the land's proceeds except the agency fee. He must execute the agency within the bounds of the authorization, provide you with information, render an account, return what belongs to you, and the expenses shall be covered from your funds. However, if he performs work other than that of the agency, he is entitled to a fair wage if you grant him permission.