It shall be the duty of the commissioners of the sinking fund during the month of July next, and in every month thereafter, when they shall have funds in the treasury belonging to the sinking fund, to receive sealed proposals for the redemption of any part of the loan of this commonwealth next becoming due; and on the first Tuesday of each month thereafter, to meet and open such proposals, and award to the bidders all or such proportions of said bills, and at such rates, as can be paid with advantage to the commonwealth, or as the conditions of the treasury, present or prospective, may justify.
72 P.S. § 3650