The certificates shall be sold by the board at their face value to state agencies having jurisdiction over special or trust funds, without advertisement or competitive bidding. Such certificates are a legal investment for any state special or trust fund notwithstanding any provision of law limiting the investment which may be made by such special or trust fund. Payment of the purchase price may be made in accordance with mutual agreement between the state agency and the board, which agreement may provide for such payment in installments as work upon the public building or buildings progresses and funds necessary therefor are required.