Thank you for your question.
I understand you are concerned about this money.
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The payments you describe unfortunately sound very familiar, and not in a good way. Using the name of the British or US Government authorities to extract rax payments in advance is a hallmark of known scams.
You may have visited your customer and his bank and seen funds on deposit. It means nothing. It's a ruse to get you to fork over more money. You likely lost all the money for the work you did and are not getting paid for. Now they are mliking you for more funds. If you were to pay the current "tax fees", you most certainly would be hit with another fee, such as the UN anti-fraud wire transfer fee or the FBI anti-terrorist fee. None of these things are real.
MI5 does not have anything to do with what this group is claiming. MI5 is British Security. It could care less about private business transactions.
Your best recourse is to contact British police or Interpol to see if they have any advice about the apprehension of these scammers.
Best of luck to you.