The Attorney General may authorize the use of a sum not to exceed $100,000 in each instance, payable from any unexpended balance of the appropriation "Support of United States prisoners" for the purpose of leasing or acquiring a site, preparation of plans, and erection of necessary buildings under section 4003 of this title.

If in any instance it shall be impossible or impracticable to secure a proper site and erect the necessary buildings within the above limitation the Attorney General may authorize the use of a sum not to exceed $10,000 in each instance, payable from any unexpended balance of the appropriation "Support of United States prisoners" for the purpose of securing options and making preliminary surveys or sketches.

Upon selection of an appropriate site the Attorney General shall submit to Congress an estimate of the cost of purchasing same and of remodeling, constructing, and equipping the necessary buildings thereon.