Is it permissible for a brother to deny his brother's right to their partnership money after 5 years, claiming that the business is losing money, while there are no documents proving the partnership?

The agent in a Mudarabah (partnership with capital and labor) is trustworthy with the money and is not liable for any losses unless caused by negligence. His claims regarding damage or loss are to be accepted, because he is managing another's money with their permission and does not exclusively benefit from it. Therefore, if the agent claims a loss and the money is gone, his word is to be taken, and his accountability, if he is lying, is with Allah.