Years ago working as a subcontractor, paying quarterly, keeping enough in savings to pay that 1099 in full every quarter they tried to shaft me. In FL paying only federal made it easier I guess, kept a little notebook with my hours, location, mileage, travel time sometimes specific events. Had copies of everything from three years back on file. Son of an accountant and a bookkeeper I'm not a whiz but I can keep up.
They tried to tell me I didn't pay enough in a hundred or so hours of overtime pay @ x1.5 pay one year. It was the most recent tax year.
It wasn't on my side though, it was on the side of the company that paid me that the error was made. Not sure how except they did hold out from me something when it went to overtime. Somehow that "helped" them, after this I knew it was shenanigans. They did not hold out enough, not my problem. I was living pretty good on $12.00/hr back then.
I held my ground waited way too long several times, the third time or after the third notice that they had sent me and I went there to ream them again that b^$tard at the Orlando office took one look at me with my paperwork in hand again, signed something on his desk, even made a copy and handed it to me. Says "Have a nice day Sir" goodbye! They acquiesced!
I knew I was right and got them to believe it too.
Your situation has the similarity of that third party not doing something right also.
Nobody f's with my money, my squeeze or my truck so I fought for it and this so-in-so won.