What should one do if they borrowed money without its owner's knowledge, intend to return it, and fear informing the owner lest they lose their trust?

Taking deposited money is reprehensible if the borrower is able to repay, and unlawful if she is unable. As for taking money that was not deposited, it is undoubtedly unlawful, because it is an aggression against another's property. The Prophet, peace and blessings be upon him, said: "Indeed, your blood, your properties, and your honor are sacred to you, like the sanctity of this day of yours, in this month of yours, in this city of yours," and "The property of a Muslim person is not lawful unless given with his willing consent." Repentance for this is obligatory, and among the conditions of repentance is returning rights to their owners, as the Prophet, peace and blessings be upon him, said: "The hand is responsible for what it took until it returns it." What is now obligatory is to return the money in any suitable way without informing him of what happened.