What is the way to deal with a manager who prevents promotion and demeans the loyal employee in front of senior management?

Sustenance is in the hands of Allah alone, and the believer must endure affliction, for patience is the key to relief. Servants possess no power to benefit or harm themselves or others. If a person performs their work with sincerity, Allah will reward them, and their recompense will not be lost. If the accusation against the manager is mere conjecture, it should be disregarded. However, if there is clear evidence of his injustice, he should be advised gently, and the matter should be elevated to a higher authority, while relying on Allah and supplicating Him, and enduring any favoritism they may encounter.