D.C. Code § 10-612
The Administrator of General Services and the Mayor of the District of Columbia are authorized to amend existing contracts and agreements by which funds have been loaned or advanced or are obligated to be loaned or advanced to said Mayor, for the acquisition, purchase, construction, establishment, and development of public works, pursuant to the authority of §§ 10-603 to 10-606, or §§ 10-607 to 10-611, so as to provide for the payment of interest on the amounts of such loans and advances to be repaid to the Administrator of General Services at such rate as would, in the opinion of the Secretary of the Treasury, be the lowest interest rate available to the District of Columbia were said District authorized by law to issue and sell obligations to the public at the par value thereof, in a sum equal to the repayable amounts of such loans and advances, maturing serially over a period of 15 years in approximately equal annual installments, including both principal and interest, and secured by a 1st pledge of and lien upon all the general-fund revenues of said District.
D.C. Code § 10-612