Should a trust concerning a sold commodity, for which the full price was not paid, be returned in cash at its old value, or at the commodity's current value? And is it considered an inheritance for the heirs, or an ongoing charity for the deceased?

The refunded amount is an inheritance to be divided among the grandfather's heirs. It should not be given as charity on his behalf unless the heirs wish to do so. The exact previous amount should be returned because money is among the fungible items, so it is returned by its like, unless the currency collapses or there is excessive deception, in which case the amount is returned by its value.