Who is responsible for paying off the debt after money was stolen from the apartment: Is it the debtor or the brother who was supposed to deliver the money?

If your brother was negligent in safeguarding the money, thereby exposing it to theft, then he is liable for its guarantee. If he was not negligent, then he is not liable for its guarantee, because the hand of an agent is a hand of trust, and he is only liable for transgression or negligence. Negligence is determined by common practice. If your brother is not liable for the guarantee, then the money remains an obligation upon you until you repay it.