Is it permissible for a person to give their money in charity to their heirs before their death instead of them inheriting it, and can they keep the will a secret until their death, and does the deceased receive reward for their lawful money that they left for the heirs?

It is permissible for a Muslim to give all or part of their wealth in charity to their heirs or to other poor and needy individuals. This is considered a sadaqah (definition) (charity) or a hiba (gift) and not a tarikah (definition) (inheritance). For this to be valid, the recipient must take full possession of the gifted item during the lifetime of the giver, or before their death or death-illness.

However, writing this down and keeping it a secret until the owner of the wealth dies is not valid, because it is considered a disposition of more than one-third after death. The Prophet, peace and blessings be upon him, said to Sa'd, "One-third, and one-third is much." Furthermore, a bequest of more than one-third is not valid, nor is a bequest to an heir.

As for the deceased receiving reward from the lawful wealth they left to their heirs, it is hoped that they will receive reward, especially if the heirs are righteous and spend the money on good deeds and acts of charity. What truly reaches the deceased as reward after their death is sadaqah jariyah (ongoing charity) and the good deeds they performed during their lifetime.