Is the new contract, which includes the two plots of land—the one purchased from the father and the one surrendered by the uncle—legally valid, and is it permissible to use it as a defense against the uncle's claim for the land he surrendered, especially since the uncle had originally sold a part of it?

If the sale was genuine, then the land is yours, and you have proof of your right. However, if the sale was fictitious—meaning you agreed with him to sign a non-genuine sales document to enable him to fence his land and benefit from it—then the land is his, and you have no right to it. It is not permissible to deceive him, as he signed the document with you to facilitate his use of his land. And whoever wrongfully takes a span of land will have it tied around his neck from seven earths on the Day of Resurrection.