If the testator orders that a thing belonging to another be acquired in order that it be given to a legatee or devisee, the heir upon whom the obligation is imposed or the estate must acquire it and give the same to the legatee or devisee; but if the owner of the thing refuses to alienate the same, or demands an excessive price therefor, the heir or the estate shall only be obliged to give the just value of the thing. (861a)