With the DIFFERENCES method, amounts can be positive or negative—negative for returns and positive when adding items to the same invoice. In this case, the correcting invoice either subtracts or adds to the original invoice, meaning both the original and the correcting invoice remain valid.
With the SUBSTITUTION method, the correcting invoice replaces the original one, and the tax agency considers only the correcting invoice valid.
Example: Final invoice should include 2 coca colas and 2 pastas
INV 2024-001 CORR 2024-001 (SUBSTITUTION method)
1 coca cola 2 coca cola
1 pasta 2 pasta
20€ taxable amount 40€ taxable amount
4,2€ VAT (21%) 8,4€ VAT (21%)
24,2€ Total 48,2€ Total
INV 2024-002 CORR 2024-002 (DIFFERENCES method)
3 coca cola -1 coca cola
3 pasta -1 pasta
60€ taxable amount 20€ taxable amount
12,6€ VAT (21%) -4,2€ VAT (21%)
72,6€ Total -24,2€ Total
Both methods are valid, and you can find examples of each in our Postman collection.
Example: Full refund
Original invoice
"full_amount": "24.20",
"items": [
{
"text": "T-shirt",
"quantity": "1",
"unit_amount": "20",
"full_amount": "24.20",
"system": {
"type": "REGULAR",
"category": {
"type": "VAT"
}
}
}
] CORRECTING invoice (SUBSTITUTION method)
"full_amount": "0",
"items": [
{
"text": "T-shirt",
"quantity": "0",
"unit_amount": "20",
"full_amount": "0",
"system": {
"type": "REGULAR",
"category": {
"type": "VAT"
}
}
}
] CORRECTING invoice (DIFFERENCES method)
"full_amount": "-24.20"
"items": [
{
"text": "T-shirt",
"quantity": "-1",
"unit_amount": "20",
"full_amount": "-24.20",
"system": {
"type": "REGULAR",
"category": {
"type": "VAT"
}
}
}
]