What is the ruling on saying, "May Allah not have mercy on him," and supplicating against the deceased at the funeral, knowing that this was said immediately upon hearing the news of death directly after the Maghrib Adhan?

If someone wrongs you, forgiving and pardoning them, especially if they are a relative, is one of the greatest acts of devotion and obedience, the fruits of which a Muslim reaps in this world and the hereafter. It is permissible to supplicate against an oppressor, whether alive or dead, provided that the supplication does not exceed the extent of the transgression; for a supplication that is greater than the transgression is considered an aggression in supplication. Thus, the wronged person may say: "O Allah, grant me justice against him." As for saying "May Allah not have mercy on him," this involves aggression. However, if you utter it without intending to, there is no blame upon you, due to the Prophet, peace and blessings be upon him, saying: "Indeed, Allah has overlooked for my Ummah mistakes, forgetfulness, and what they are compelled to do."