Discussion - Agency

David is a purchasing agent for ABC Corporation. Andy, a ABC corporate officer, gives David written authority to buy for the firm as many Apple devices as necessary. The next day, Andy calls David and tells him to buy only fifty iPads and nothing else. David shows the written authority to XYZ, Inc., and enters into a contract with XYZ to buy sixty iPads and a selection of iPods. XYZ ships the order to ABC. Is ABC liable to XYZ under the contract? Is David liable? In each case, if so, why? If not, why not?