Is it true that it is impermissible to expose a Muslim, even if he is a thief, and what is the ruling of Islamic law on someone who falsely accuses a Muslim openly, and on the reaction to that?

If there is clear evidence of the center director's theft, and he refuses to stop after being advised, then it is obligatory to inform the members who are capable of stopping him, acting upon the Prophet's saying, "Support your brother whether he is an oppressor or oppressed," and "Whoever among you sees an evil, let him change it." As for concealing his actions, it is impermissible if it results in harm to Muslims and squandering their funds. However, the desirable form of concealment is to dismiss him and hold him accountable without exposing him publicly, as that might damage the reputation of Islamic work. Lawful concealment applies specifically to someone who is not known for causing harm and whose sin does not extend to harming others. As for pushing the Khatib (preacher) and stopping him from delivering the sermon, it is impermissible because it opens the door to discord in the mosque. The preferred approach would have been to wait until the sermon ended and then respond and defend oneself. We advise everyone to uphold justice and reconcile differences, acting upon the Almighty's saying, "O you who have believed, be persistently Qawwameen (firm upholders) for Allah, witnesses in justice," and "So fear Allah and amend that which is between you."