Withdrawal of accepted manuscript from predatory journal
Get legal advice. I don't see any other way about this unfortunately. They could easily say the 1819 EUR charge was a typo and it should've been 1819 USD. It also clearly says on the website that there's a withdrawal charge, although it doesn't say how much. They have removed the reason for the withdrawal charge from their website; however I'm sure I saw in previous versions of the page that the reason is you've taken up the editors' + reviewers' time, which is a defensible argument.

You may have to provide an explanation for why you submitted an article to them in the first place. If you can e.g. point to a deceptive call for papers which did not mention the article processing charge, you have a stronger case.

Having said that, don't worry too much, you're probably going to get away without too much pain. Reasons:


The publisher is OMICS, which has a reputation as the worst predatory open access publisher out there (see their Wikipedia page). You can point to action by the US government against OMICS, which should be especially relevant if you're based in the US.
Because OMICS is so bad, there're likely to have been many authors who wanted to withdraw their articles. If OMICS has initiated legal proceedings against these authors, I'm not aware of them (and they would undoubtedly have been publicized by the victims).
OMICS has already been involved in lots of legal suits, on the wrong side (i.e. they're the defendants). Courts are not likely to be sympathetic to predatory OA publishers.
OMICS has little to gain by filing a suit. The most they can hope for is 1819 USD. That's probably not enough to be worth the effort. 


I'd guess that the most likely result is that you'll be blacklisted by OMICS. They might refuse to consider any more of your papers or they might charge higher article processing charges the next time you submit an article. They might even take you off their mailing lists. Do you care? One might even send them some genuine thanks if they do.