Not sure what to recommend but you should definitely get this decision from them in writing. If it is only verbal from a tax officer who is unwilling to issue a formal letter it is a shakedown, if it is in writing they have to reference the specific law or regulation, showing their calculation of the amount requested, and then you can consider how to proceed and whether you need to use a tax advisor (bearing in mind that will cost money too and might not change the result). In any case a tax advisor can't do much until there is something in writing.
Hopefully they don't ask you to pay a smaller amount to "make the problem go away" or similar, which is not unusual in certain govt bodies, and of course they'd be back for more later.