بسم الله الرحمن الرحيم
If imam sahib has repented and his beard has grown one fist or longer as well as the management is satisfied with his taubah then the said person may be appointed on the post of imam again and salah will be valid behind such person.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.