What should we do regarding a mosque muezzin who collects donations but refuses transparency and disclosure about the fate of the funds, and threatens whoever asks him about that?

Funds from donations and charities must be managed in accordance with the donor's stipulations and the best interest of the charity. Only trustworthy and reliable individuals should handle this. If the muezzin is trustworthy, then a good opinion should be held of him. However, if suspicion arises, the matter must be investigated without harming anyone's reputation. The matter should be reviewed by the mosque administration if it is officially affiliated, or by the mosque imam, or by the congregation. If the muezzin's betrayal is proven and the matter cannot be rectified, then worshippers must be warned against donating to this mosque.