"This report reviews the process and procedures that currently apply to congressional consideration of foreign arms sales proposed by the President. This includes consideration of proposals to sell major defense equipment, defense articles and services, or the re-transfer to third party states of such military items. Under Section 36(b) of the Arms Export Control Act (AECA), Congress must be formally notified 30 calendar days before the Administration can take the final steps to conclude a government-to-government foreign military sale of major defense equipment valued at $14 million or more, defense articles or services valued at $50 million or more, or design and construction services valued at $200 million or more. […] In general, the executive branch, after complying with the terms of applicable U.S. law, principally contained in the AECA, is free to proceed with an arms sales proposal unless Congress passes legislation prohibiting or modifying the proposed sale. Under current law Congress faces two fundamental obstacles to block or modify a presidential sale of military equipment: it must pass legislation expressing its will on the sale, and it must be capable of overriding a presumptive presidential veto of such legislation. Congress, however, is free to pass legislation to block or modify an arms sale 'at any time' up to the point of delivery of the items involved."
CRS Report for Congress, RL31675
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html