Tax fraud is apparently prosecuted and punished more harshly than manslaughter. I would rather drive drunk than cheat the tax authorities.
However: creative interpretation is not fraud. Not reporting something when you are supposed to is obviously not allowed. But it is debatable whether a washbasin is an essential part of the property or should be considered like a wardrobe or bed. In that case, no real estate transfer tax would be due on it.
I would deal openly with the tax office, send them the invoice, and immediately clarify what is part of the property and what is not. Then just wait.
Also interesting in this case is whether you have already paid VAT on the washbasin. Then you would have to get it back from the tax office if the washbasin is subject to real estate transfer tax.
Did the developer include the VAT in the itemization?