All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
The woman's mahr is her own property, and she is entitled to dispose of it as she wishes in any lawful manner.
Allaah says (what means): {And give the women [upon marriage] their [bridal] gifts graciously. But if they give up willingly to you anything of it, then take it in satisfaction and ease.} [Quran 4:4]
The verse indicates that it is not permissible for a husband or anyone else to take anything from his wife’s mahr except with her own proper consent.
Therefore, if your grandfather had given your grandmother that property as a mahr for her, then she has the right to dispose of it as she wishes, and he is not entitled to take it back.
Perhaps your grandfather does not admit having given the property to your grandmother and claims that it is still his property. If this is the case, then you must take the case to an Islamic judge or his representative because then the matter would have become a dispute over a claim, which requires to be taken to court.
Allaah knows best.