Is it permissible for an employee or a manager to inform the employer of mistakes, even if they were discovered before they occurred? And am I entitled to reclaim the money I gave to my colleague?

If espionage is not part of a person's job, then it is not permissible for him to spy on you, track your mistakes, or inform on you, unless he sees a mistake that harms the work, in which case he should advise you. If you do not desist, then he may report to the responsible authority. Gifts given to overlook mistakes are forbidden bribery, and you have the right to demand their return, and you must repent from that. Allah Almighty has forbidden consuming people's wealth unjustly, such as usury, bribes, gifts to workers, and theft. The wealth of a Muslim is inviolable. Whoever takes people's wealth unjustly will return it sooner or later, so he should fear Allah and free himself from people's grievances before it is too late.