A Jew hired a worker to work for him on Shabbat, forgetting that he’d be working on Shabbat. What should he do?

Imagine someone needs a person to help him build a website. He found a website which offers the services he needed for very reasonable prices. He found a guy, hired him, all without realizing that he’d be working for a Jew and finishing the project on Shabbat. With the service provider's system, he's pretty much paid already, just the money doesn’t go to the worker till the job is done.

It’s probably possible to ask him to postpone the work, but then again his deadline to be paid falls on Shabbat, and the man hiring him doesn’t know what his response would be to that. What is the proper thing to do, according to halachic sources? Theoretically, does it make a difference is the worker is likely to be a Jew or a non-Jew?