What is the ruling of Islamic law regarding guaranteeing money when participating in a Mudarabah that turns out to be fraudulent, knowing that one of the partners collected a larger sum, and does the agent guarantee the money if the Mudarabah involves risk?

If the questioner and his brother took money for a Mudarabah (profit-sharing) partnership without the knowledge of its owners, and then gave it to a third party who betrayed them, then each of them must guarantee the money they took and return it to its owners. This is because it is not permissible to engage in Mudarabah with people's money without their permission. If one does so, he is like a usurper.