D.C. Code § 10-904: Mayor authorized to pay or receive amounts as part of consideration for exchange.

If, in the opinion of the Mayor of the District of Columbia, the value of the land to be conveyed to the District is in excess of the value of the land to be conveyed by the District, the Mayor is authorized to pay, within the limitation of appropriations therefor, to the abutting property owner the amount of such excess as determined by the Mayor, on the basis of an appraisal, and, if the value of the land to be conveyed by the District is in excess of the value of the land to be conveyed to the District, the Mayor shall require the abutting property owner to pay such excess as determined by the Mayor, on the basis of an appraisal, as part of the consideration for the said exchange.