What is the amount of blood money (diyyah) for a man and a woman, and what is the ruling of Islamic law if the killer's family is unable to pay it? Does any harm befall the deceased from this?

Regarding the second question, the heirs are not at fault for demanding the blood money (diyah), nor is there any sin upon the deceased if his guardians do so. However, forgiveness, pardon, and overlooking for the sake of Allah are better, as Allah Almighty says: "So whoever pardons and makes reconciliation - his reward is due from Allah," and "and those who restrain anger and pardon people." The blood money in unintentional killing is to be paid by the 'aqilah (the male relatives of the offender on his father's side), and if they are not found, then from the public treasury (Bayt al-Mal), and if that is not found, then from the perpetrator's own money. And indeed, Allah has encouraged forgiveness and pardon.