This actually happened to my wife but she's under too much stress to deal with it and I'm not a citizen so I'm less sure what to do.
Timeline: In October she did an interview, having already booked flights to leave for 10 work days over Christmas, and in the interview it was agreed that this time could be taken as PTO. In orientation when the job started (a week or two later) it was stated that PTO had to be accrued, at a rate of 8 hours per two months, but we we're under the impression that the agreed PTO was a condition of hiring. On Nov 27th we have an email stating that PTO was approved for the given date. Jan 4th was payday and there was no pay.
The Chief Medical Officer (this is a private healthcare job and she's the only one that could approve PTO) said "why would I approve that in the interview?" and when confronted with the email said it was a "terminology error".
What can we do? For info these wages would be about $950 after tax so we're willing to do a decent bit to get this back. In terms of resources only her parents and sister are in the country (and aren't that headstrong), I'm a STEM PhD student and may or may not have access to legal resources through my university but it's too hard for me to check because I'm flying right now.