Is it permissible for me to take a commission from the sale of my cousin's land if an agreement was made with him on a specific price, and the remaining increase is for me and the brokers, without informing him of the real selling price? And can I give the amount in charity on his behalf if it is impossible to return it?

If the owner tells the agent, "Sell it," and the agent sells it for more than the price specified by the principal, then the entire price belongs to the owner. The agent is only entitled to a fair wage (ujrat al-mithl). Agency (wikalah) is a contract of benevolence ( عقود الإرفاق), and the agent is not entitled to a wage unless it is stipulated or customary. So, if custom dictates that the agent receives a wage, and the fair wage is equal to or greater than the increase, the agent may keep it. If the wage is less, the agent may not keep the surplus or donate it on behalf of the owner. Instead, the owner must be asked for permission, or the surplus must be given to the owner without informing him that it is from the sale price of the land.