N.Y.C. Charter § 294: Section 294. Redemption.

The sinking fund bonds to be redeemed from the general sinking fund or any additional sinking funds established pursuant to section two hundred ninety-eight may be selected in such manner as the comptroller may determine and may be identified on the face thereof. The principal amount of sinking fund bonds required to be redeemed on any date by payment from the general sinking fund or any additional sinking fund shall be reduced by the principal amount of any such bonds which has been timely purchased or redeemed and cancelled by the city and not theretofore applied as a credit against such requirements.