How to use the tax_base field for special VAT regimes?

Tip

Last updated on 2025-07-24.

In some special VAT regimes—such as Antiques (REBU) — VAT is charged only on the profit margin, not on the full selling price. When this happens, the normal formula unit_amount × quantity doesn’t match the taxable base, so you must specify that base explicitly with the tax_base field.

Let’s take this example:

Purchase price	400€	What you paid for the item
Selling price	600€	What the customer pays
Profit margin	200€	600 - 400 = 200
VAT rate	21%	VAT rate
VAT due	42€	21% of 200 = 42
tax_base to report	200€	Margin subject to VAT

How does this look like in the SIGN ES API?

full_amount = (unit_amount × quantity) + VAT Here, VAT is 21 % of the margin (€200), not of the selling price.

 

Example payload:  

"content": {
        "type": "SIMPLIFIED",
        "number": "12",
        "series": "E",
        "text": "example invoice",
        "full_amount": "642.00",
        "items": [
            {
                "text": "Mueble",
                "quantity": "1",
                "unit_amount": "600",
                "full_amount": "642.00",
                "system": {
                    "type": "ANTIQUES",
                    "tax_base": "200.00",
                    "category": {
                        "type": "VAT"
                    }
                }
            }
        ]
    }