What is the ruling on money taken from a company six years ago, given that the amount is not remembered and there is doubt about its return, and is it permissible to give it as charity if it is impossible to return it?

Taking people's money unjustly is a grave sin, especially if the one who takes it was entrusted with it. Repentance and returning the rights to their owners are obligatory. If you know that the company has rights owed by you, then you must return them, even if through indirect means. However, if it is merely a suspicion, you are not obliged to return it, but the more cautious approach is to act to clear your conscience.