(a) The comptroller may not register any refunding bonds until the original bonds for which the refunding bonds are issued are presented to him for cancellation or until a contract for the purchase of a corresponding number of the original bonds has been entered into and filed with the comptroller.
(b) After the refunding bonds are registered, the comptroller shall keep them in his possession until the original bonds are surrendered to him and cancelled by him, at which time he shall deliver the new bonds to the proper party or parties.

(c) The original bonds may be presented for payment in installments and an equal amount of refunding bonds registered and delivered as provided in this subchapter.