Is it permissible to donate the monetary value of what was stolen from the grandfather and grandmother after their death, due to the difficulty of reaching the heirs, and with the intention of conveying the reward of the donation to them?

For a repentance related to the rights of people to be valid, it is a condition to return injustices to their owners or to seek their absolution. So, whoever has wronged his brother should seek his absolution today, before there is no dinar nor dirham. If a person stole another's money and finds it difficult to inform him, or fears an increase in mischief, he is not obliged to inform him. Rather, he should return the money by any possible means. The questioner must return the money to the heirs of his grandfather and grandmother as long as it is possible, for the money is no longer the property of the grandfather and grandmother, but has become the property of their heirs. It is not permissible to give charity from someone else's money without their knowledge. If a person fears Allah Almighty and is keen on fulfilling rights, Allah will make it easy for him.