Notwithstanding any other provision of law, the head of any department or other agency in the executive branch of the Government which subsequent to September 9, 1939, entered into any contract or agreement with the holder of any privately owned patent or any right thereunder whereby such holder granted to the United States, without payment of royalty or with reduction or limitation of royalty, any license under such patent or right, is authorized, upon application of the grantor of such license, to enter into such supplemental contract or agreement for the cancellation of the contract or agreement by which such license was granted as the head of such department or agency shall deem to be warranted by equities existing by reason of changes in circumstances occurring since the granting of such license.