How much are we talking about? It's very brazen to put that out when everything is in order. Basically, it's a kind of extortion. If you missed something is one thing. Aren't they afraid for their jobs?
You may need a recording if you want to go further (
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). However, you may need to need to consider how it effects relatives etc.
That you don't mention actual amount is a little bit "funny".
I'm not sure if you can just go to Bangkok and do the extention there, but it may be something to reaearch if you know for sure everything is ok...
I'd think the "tourist" police would be meant for this,
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but I'm not sure what they'd actually do anything, noticing that their register is all in Thai... Meaning is just PR and no information? They are supposed to help you with "problems", maybe one of them will go to immigration with you? Lol
I'd suggest that there maybe something missing and they are "trying to help you" with something... Maybe I'm just naive.
Maybe just smile more and not move from the chair, and be polite.