Is it permissible to take a personal loan from an imam, knowing that this money belongs to the mosque and is not the property of the imam, and that the imam justified giving it to trusted individuals for fear of theft or accountability, and that this financial surplus resulted from an abundance of charitable donations? And if it is not permissible, what is the repentance for it?

It is not permissible for the Imam of a mosque to lend money donated to the mosque or for charity, because he is an agent for the donors, and an agent is bound by the permission of his principal. Unless the donors permit lending, he is not allowed to do so. If the money is donated to the mosque, it is an endowment (waqf), and jurists have stipulated that lending from an endowment is prohibited. Therefore, the Imam erred in this action and is considered a usurper and guarantor of the money. You also erred by taking this loan, knowing that the lender does not own it. Since you have returned the money, nothing is required of either of you but repentance. However, the Imam must be cautioned about his mistake in taking money for the mosque beyond its needs, and then his mistake in lending this money or withholding it from those who are entitled to it.