Suppose someone hires a worker through an online intermediary such as Fiverr to do a discrete project, such as building a website, and inadvertently sets the deadline for project completion to Shabbat, such that the worker will likely actually do the work on Shabbat. Must the hire-er try to stop the worker from working on Shabbat? What is the proper thing to do, according to halachic sources? Does it make a difference if the worker is likely to be a Jew or a non-Jew?
If you can stop him from working on shabbat, you should do so and require that no work be done on shabbat if he's Jewish. If he is Jewish and already finished the site after the fact, contact your local orthodox Rabbi about whether it is permitted to benefit from the site.
If the worker is not Jewish, there is no need to stipulate such if he is paid per job and it was awarded to him prior to the start of shabbat and he could theoretically finish before shabbat. If the case was such that he did finish it on shabbat, it is permitted to benefit from the site since the non-Jew undertook to finish the job for his own benefit to receive payment and could have finished before shabbat(the fact that he actually finished it on shabbat was due to his own comfort and decision and not because you told him to do so on shabbat itself). See Yalkut Yosef Shabbat part 1 pg. 29 and part 2 pg. 628, and Halichot Olam part 4.