Is it permissible for a father to take possession of a piece of land whose owners guaranteed him and wrote it over to him with fictitious contracts as compensation for a previous inheritance, while there is a legitimate heir, and is it permissible for the son to take his share of this land after the father's death?

Your paternal aunt's donation of the plot of land to your father has two scenarios:

The first: That the donation was effective during her lifetime, and your father accepted and took possession of the land. This is a valid gift (hiba), and the heirs are not permitted to claim it back.

The second: That the donation was contingent upon her death. This is a will (wasiyya), and your father is not permitted to take more than one-third of the estate through it. If it exceeds one-third, the remainder must be returned to the heir.

Furthermore, your father has a share in the estate, amounting to one qirat, because he is a partner in the usurpation (ghasb). Her cousin must return to your father what he took from him of the land and whatever fruits or rent resulted from it.

Accordingly, you are permitted to take your share of this land after your father's death, provided that what he took was within the limits of his aforementioned right.