“At one time, this gave rise to the question of whether the legal validity of the money was determined by the stamp of the ruler of the country or by the metal content of the coin; later, to the question of whether the command of the law or the free usage of business was to settle if the money was legal tender or not. The answer of public opinion, grounded on the principles of private property and the protection of acquired rights, ran the same in both cases: Prout quidque eontraetum est, ita et solvi debet; ut cum re eontraximus, re solvi debet, veluti cum mutuum dedimus, ut Tetro pecuniae tantundem solvi debeat.”