What is the ruling regarding blood money for a poor person who cannot pay it, and the family of the deceased does not accept an amnesty, and how can he pay it and clear his conscience?

If the full blood money is not available, it becomes a debt owed to the family of the slain, and they must grant you respite because you are in hardship. It is permissible for you to take from the Zakat to pay off the blood money, because you are among the debtors. If you die in debt, then it is incumbent upon the ruler to pay off your debt from the treasury of the Muslims.