1. Since the allotment was made prior to partition, the subsequent partition allotting BDA property to G only would make the allotment beyond any dispute anymore.The other legal heirs of X by way of partition deed gives a seal of approval to the allotment to G by BDA.
2. The allotment to X having been his self acquired property, his grand children acquires no share in it on the basis of inheritance.
3. So in other words you can safely purchase this property.