The issuer may at any time instruct the State Comptroller to purchase sinking fund bonds with amounts on deposit in the sinking fund account established for the redemption of such bonds. The issuer shall specify the amount to be used to purchase such bonds and the maximum price payable, and shall agree to pay to the State Comptroller on or before the date of purchase of any costs or expenses associated with such purchase, including without limitation any brokerage fees or charges; provided, however, that, in the event such costs and expenses cannot be determined on or before such purchase date, they shall be paid within 15 days thereafter. The State Comptroller is obligated only to use his best efforts to purchase such bonds.