73:2 The second condition that must be met in order for a non-Jew to do labor for a Jew on Shabbos is that the non-Jewish laborer must have set wages pre-established. In this way, the non-Jew is doing the work for himself, in order to earn his pay. Therefore, if a Jew has a non-Jewish worker only for a certain amount of time, he may not allow him to work on Shabbos because the work is then being performed solely for the Jew’s benefit. If a non-Jew is traveling somewhere and a Jew gives him a letter to deliver there, if the non-Jew will also be traveling on Shabbos, the Jew must pay him a fee for his trouble. In this way, the non-Jew is doing it for himself, in order to earn his pay, and not as a favor for the Jew per se. (If the non-Jew volunteers on his own initiative to deliver the letter, it is permitted to allow him to do so even for free – Rema 247:4.)
73:3 The third condition is that the non-Jew’s wages must be a flat fee for the whole job and not a certain amount for each day by itself.