Is the money taken by the young man without knowledge of the officials' authorities permissible, or must it be returned? And does the obligation to return it cease if the officials refuse to take it back?

If the law does not permit granting that stipend to someone who has left the country, and the employee violated the law by giving it to you, then it is not lawful for you. And if you do not know whether the law permits it or not, then employees usually abide by laws and regulations for fear of penalties, so it is unlikely that they would pay money to someone who is not entitled to it according to their regulations. If that is the case, then do not demand the return of the money.