What is the extent of the sin resulting from slandering a virtuous manager, which led to his dismissal and caused him and his family psychological harm, and is there repentance for that?

The first step is to reciprocate kindness with kindness and to advise the wrongdoer, while concealing his faults as much as possible. What happened is not a sin, provided you did not fabricate lies against him, but you missed out on the virtue of concealing faults. We advise you to mend this man's broken spirit, improve his image, mention his good qualities, and explain that what happened was a minor slip, and that his transgression was a mistake that needed to be spoken of. We ask Allah to accept his repentance after the stolen items are returned. Let him praise Allah for His well-being, for the disgrace of this world is lighter than the shame of the Hereafter, and the talk of people will not harm him after his repentance. Increase your supplications for his forgiveness and strive to win over his heart, and express your regret if he learns that you were the cause.