Is the mosque’s trustee obligated to pay the 750 EGP that was stolen from him while he was on his way to a government agency to complete the procedures for connecting water to the mosque, or does the mosque bear that [cost]?

If the person in question has not been negligent or exceeded their bounds, then they do not guarantee this money, because they are a trustworthy agent unless they have been negligent. Ibn Qudamah said in Al-Mughni: "If the agent sells and then claims the price was destroyed without exceeding their authority, there is no guarantee upon them. If they are suspected, they should take an oath."