The Secretary of Agriculture is hereby authorized to accept, receive, hold, and administer on behalf of the United States gifts, bequests, or devises of real and personal property made unconditionally for the benefit of the National Agricultural Library or for the carrying out of any of its functions. Conditional gifts may be accepted and used in accordance with their provisions provided that no gift may be accepted which is conditioned on any expenditure not to be met therefrom or from the income thereof unless such expenditure has been approved by Act of Congress.