A guinea was an old gold coin, worth slightly more than one pound sterling (21 shillings - 1.05 pound). The coin itself was not in circulation at that time but it was still used as a unit in betting on horses and in "aristocratic" contracts. Joyce himself made a point of borrowing in gentlemanly guineas, and so does Stephen. The subtlety of a contract in guineas being more upper class is also present in "A Mother" in Dubliners.
