No association shall carry any real estate on its books at a sum in excess of the total amount invested by the association on account of the real estate, including advances, costs and improvements. Any association selling real estate under a contract of sale may carry the amount due the association under terms of the contract as an asset upon its books, but at no time shall the contract be considered as having an asset value greater in amount than the remaining principal balance of the contract, or greater in amount than the value at which the property so sold was permitted to be carried upon the books of the association.