Well, it's not obvious to me, if all the paperwork was in 100% good order, what "help" was being offered, what reason did the immigration give exactly?
If I knew all my paperwork was 100% legit and the person working the desk wanted to try and say I need to pay more money, I'd point out my paperwork is 100% good, no extra payments needed, esp "very large extra requirements", and in a polite but tactful manner ask for the reason, pin that worker down to a reason and go from there.