Is it permissible to buy land to build a mosque on it if this land was among those confiscated during the era of Gamal Abdel Nasser, then a farmer reclaimed it and acquired ownership from the state, and now the heirs of the original owners are claiming it judicially, which has made its price very cheap?

This land belongs to the heirs of the original owners from whom it was unjustly seized. It is not permissible for anyone other than the heirs to sell, buy, or dispose of it, even for building a mosque. The land must be vacated for the heirs. If they wish to sell it, there is no harm in purchasing it from them at a price they accept, whether for building a mosque or for other purposes.