Although the Department of Defense, through the Defense Security Cooperation Agency (DSCA), is responsible for implementing individual FMS cases, the Department of State must first review and approve them. DSCA forwards all FMS cases to PM/RSAT, which is responsible for ensuring that they are properly reviewed within the Department for consistency with U.S. foreign policy and national security objectives. In close coordination with other Department bureaus and offices, PM/RSAT officers provide recommendations to PM leadership on whether to approve the potential sale of military equipment to foreign governments through FMS.
http://m.state.gov/mc14021.htm
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 must overcome 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.
http://www.dsca.mil/resources/arms-sales-congressional-review-process