A stockholder is liable to his corporation for the unpaid portion of the par value of his stock. The directors of a corporation may make a contract with purchasers of stock to sell them the stock of the corporation at less than its par value. This contract will be binding as far as the corporation is concerned. A corporation can sue a stockholder on any indebtedness to the corporation which he may have contracted, the same as an outsider. A stockholder can set off any dividend due him against any debt which he may owe to the corporation. A stockholder cannot, however, set off a debt which the corporation may owe him against the unpaid amount due on his stock.