Is it permissible, Islamically, to resort to the judiciary to reclaim a plot of land whose donor revoked their gift after it had been taken possession of, and is there a sin incurred by reclaiming it through the force of positive law?

Selling land without specifying its price renders the contract invalid due to ambiguity. Gifting land is only permissible under two conditions: that it is given by the donor's free choice, and that the land is legally possessed (transferred). If these conditions are met, the gift becomes binding and cannot be revoked. If the donor or their heirs refuse (to complete the transfer), one can resort to the judiciary, or seek the mediation of benevolent individuals to reconcile the matter.