The President may, from time to time, when he deems it in the interest of national defense, authorize the head of any department or agency of the Government, to enter into contracts for the procurement of defense articles, information, or services for the government of any country whose defense the President deems vital to the defense of the United States, to the extent that such government agrees to pay the United States for such defense articles, information, or services prior to the receipt thereof and to make such payments from time to time as the President may require to protect the interests of the United States; and, upon payment of the full cost, the President may dispose of such articles, information, or services to such government: , That the total amount of the outstanding contracts under this section, less the amounts which have been paid to the United States under such contracts, shall at no time exceed $600,000,000.Provided