What should one do who has wrongfully withheld a portion of employees' salaries and wishes to repent?

To rid yourself of the rights of employees whom you wronged, refer to the records in your previous workplace to identify them, then contact them and seek their forgiveness. If they do not forgive you, reconcile with them on terms you both agree upon. If that is not possible, then give in charity what you believe, most likely, you took from them, intending thereby to free yourself from sin, not to draw closer to Allah; for Allah is good and accepts only what is good.