For impending FMS sales, the Defense Security Cooperation Agency (DSCA) — a branch of the US State Department — must notify Congress, giving the legislature the opportunity to question the sale.
The second category is direct commercial sales (DCS), in which the foreign customer negotiates directly with the US vendor company, with the permission of the US government. No Congressional notification is required for this.
The case of the AH-64E Apache is a particularly complicated one: the basic helicopters, including the airframe, is a DCS, which New Delhi has negotiated directly with Boeing. However, the engines, fire control systems and radars, key avionics and weapons and missiles are being bought under FMS.