In all cases where aliens have purchased real estate within this Commonwealth, and have sold the same to Citizens of the United States, the said sales shall be valid, to all intents and purposes, and shall be construed to vest the title to the said real estate, in the citizens aforesaid, as fully and effectually as if the said aliens had been citizens at the time of purchasing the same: Provided, this act shall not apply to any cases that have already been adjudicated.