Is Zakat considered complete or deficient if part of it was given to a indebted brother without his knowledge that it was Zakat money, and then the brother returned half the amount after a year, and what should be done with the returned amount?

If your brother is indebted and eligible to receive Zakat, then the Zakat has been placed in its proper recipient. You have done well by not informing him that the money is from Zakat, as silence about it is permissible. If he returns the money: it is not permissible for you to take it back. You may use dissimulation (tawriyah) by saying, "I have given this money for the sake of Allah with a willing heart, and I will not take it back." Or, you can return it to him indirectly as a gift, or by settling some of his outstanding debts. If these methods do not succeed, then inform him that it is Zakat and cannot be returned. If he refuses it after knowing, then give it to other poor people.