N.Y.C. Admin. Code § 5-304: § 5-304 Cession to and purchase by the city of the real property being acquired.

a. An owner of real property, which property the city is authorized to acquire, may cede the same to the city upon such terms and conditions, including exemptions from assessments, as the mayor from time to time may prescribe, provided such real property be free from encumbrances inconsistent with the title to be acquired.

b. The mayor shall also have power and is hereby authorized to agree as to the purchase price of any real property selected for acquisition in a capital project proceeding, or any part thereof, and to purchase the same for and on behalf of the city. An option to purchase such real property, granted to the city for a period not to exceed ninety days, shall not be withdrawn or cancelled during the period named therein.

c. When a conveyance of the real property ceded or purchased shall have been approved and accepted, the city shall become vested with title to such real property so conveyed to the same extent and effect as if it had been acquired for the improvement by a proceeding had for that purpose.