My boss won’t pay me.

So I work for USPS (the post office). I worked on thanksgiving for 12 hours at a plant which would have given me 12 hours of holiday pay plus 2 hours of overtime (after 8 hrs) plus 2 hours of penalty overtime which is after 10. So all together I should have made around 206 for 8 + 77$ for 2 hours plus $103 for the last 2 hours. Equaling 386$ roughly. Just for that day. However it has been 4 months almost now and still no pay for it. I’ve reminded him countless times and he said he’ll fix it. The union has reminded him many times and nothing. Just this week he said that I can’t get the pay unless I stay under 40 hours for this week. Which is wrong because it’s back pay. And that is an adjustment which has a totally separate area of the check. He’s saying that he only owes me 8 hours of reg pay. And that I got the rest . Which I didn’t. I’ve had other employees and supervisors check. To add to this, I wanted more hours because I only get around 30 here. He said he was going to send me or another girl (both in lower 20s) to another office where I was getting around 50 hours a week just by working 3 days there and the rest here. He said he had no choice in the decision who was picked. 3 supervisors at that other office wanted me. He (my boss) sent the other girl. Here’s my proof of his lie in that decision. He allowed me to get overtime the past month at the other office and then said that it “looked bad from a business standpoint”. Others have caught him looking at me in a suggestive way. He’s told others that he thinks I’m cute. (He’s in his 50s). If I get fired from this job I can never work in another government agency again. I’m in my 3 year of college to work in a government agency (law enforcement). I’ve contacted the union so what do I do? Anyone here also work for Usps? Or know the law? Labor law for my state is that any unpaid wages is due the immediate check and we’ve gotten roughly 7-8 since then. I’m afraid of getting fired but $400 is a lot to just give up on. Or have my hours cut for no reason but to save money. He also told me that when the person was sent to the other office, our hours would go up. Meaning I wouldn’t have a problem with not many hours. But what’s he do? Hired a new person to replace her 🙃. He’s blocked any transfer I’ve put in and I’m literally trapped unless I quit. Which I can’t do. It took 2 years of trying to get a job here