Am I obligated to offer a sacrifice (hady), and where should it be slaughtered, after I intended to perform Hajj al-Qiran and was not aware of the obligation of the hady until after completing the rituals of Hajj and returning?

The time for sacrificing the Udhiyah (sacrificial animal) and the Hady (offering) extends for four days: the Day of Nahr (slaughtering) and the three days following it. The place for sacrificing the obligatory Hady is the Haram (sacred precinct), and its meat is distributed among the poor of the Haram. If someone misses slaughtering it at its prescribed time, it remains a debt upon them, and they are obligated to slaughter it in the Haram. If that is not possible, then it is permissible for them to slaughter it in the place where they are.