Does it matter? Are you going to tell him “since you’re only doing it because you’re getting paid, don’t keep Shabbos?”
The mitzvah is to not do melachah on Shabbos. I don’t see why the reason makes a difference. It doesn’t matter if you didn’t do melachah because you were paid not to, because you didn’t want to, or because you were tied up in the basement all Shabbos and unable to do any. The bottom line is that no melachah was done.
The Wolf