All such refunding bonds may be exchanged dollar for dollar for the bonds to be refunded, or they may be sold as directed by the state board of finance, and the proceeds thereof shall be applied only to the purpose for which said refunding bonds were issued. When refunding bonds are sold the proceeds shall be deposited in the proper sinking fund, and the state treasurer shall immediately transfer the same to the bank in the city of New York, state of New York, designated as paying agent of the issue maturing or called for redemption prior to maturity, to be held by such bank and to be applied solely to the payment of the principal of and matured interest upon the bonds maturing or called for payment prior to maturity; provided, that any such funds remaining in the bank in the city of New York, state of New York, named as paying agent, at the expiration of two years from the date fixed by the call as date of payment of said bonds shall be returned to the state treasurer and by him held in a special fund for such purposes.