The board of directors of any irrigation district, after a sale and disposal of any lands purchased under this chapter, having contracts or evidence of lien on the lands so sold for the purchase price or any part thereof may use such contracts or evidence of lien and pledge the same to the United States or any contractor for the construction of any works necessary for the operation of the irrigation works of such district. And the board of directors of any irrigation district having such contracts or evidence of lien, may borrow money to be used for the construction of any irrigation or drainage works necessary for the use of said district, and may pledge such contracts or evidence of lien as security for such loan: provided, that no such loan shall exceed the face value exclusive of interest of the contracts or evidences of lien so pledged: and provided further, that any money so borrowed shall be placed in a special fund and used only for the construction of such works.