The December advance has been abolished.

More information can be found on this website:

Abolition of the December advance on VAT and withholding tax. The December advance in VAT will be abolished permanently as of December 2021. 

This already happened once in December 2020. The December advance payment in the advance payment on professional income tax (Art. 412, para. 4 and 5, ITC 92) will also be abolished.

Did you send the declaration anyway?

As the XML for October + November has already been created, you must delete this transmission.

This will free up the income lines for October + November so that they can be included in a subsequent declaration (for the entire 4th quarter).

If you have already sent the customers a mail with a payment order, it is best to inform them that they do not have to pay it. The structured communication is not going to be correct either, because it is made for the period of the advance payment.

Calculate and declare the withholding tax in full for the 4th quarter:

This is already foreseen in Belcofin for the declaration. When creating the XML, you can already indicate that you want to include the advance payment + December in 1 declaration.

Select at Quarterly : Advance + 4th quarter (Oct+Nov+Dec)