What is the lawful way to remove a resident volunteer at the mosque who steals from its donations, and what is the ruling on remaining silent about that?

If you are certain, based on evidence, that this person is a betrayer and embezzles donation funds, then it is not permissible to keep him in his position, for the Prophet (peace be upon him) said: "When honesty is lost, then await the Hour." It was asked: "How will it be lost, O Messenger of Allah?" He said: "When authority is given to those who are not worthy of it, then await the Hour." The members of the association can confront him with the evidence and advise him, for the Prophet (peace be upon him) said: "Religion is sincere advice." However, if the accusation is based on mere suspicions, then it is not permissible to act upon it, for Allah the Almighty said: "O you who have believed, avoid much [negative] assumption. Indeed, some assumption is sin," and the Prophet (peace be upon him) said: "Beware of suspicion, for suspicion is the falsest of speech." Nevertheless, there is no harm in taking precautions with the mosque's funds and increasing vigilance in their safekeeping without accusing anyone.