What is the ruling on a niqabi woman working in a company with a limited number of employees, and her remaining alone with the manager or one of her colleagues, due to her need for money? And is there sin upon her for that, and how can she repent from it?

If you were subjected to suspicious seclusion or intermingling with foreign (non-mahram (definition)) men in your workplace, then this is impermissible, and you must repent from it. Repentance is achieved by abandoning the sin, regretting it, and resolving never to return to it.