# PEPPOL-EN16931-R051: Use the invoice currency code in every amount currencyID

An amount has a `currencyID` other than `cbc:DocumentCurrencyCode`. All amounts must be in the invoice currency, bar the VAT total in an accounting currency.

- Layer: Peppol BIS Billing
- Severity: fatal (the document is invalid)
- Topics: Core fields
- Official definition: https://docs.peppol.eu/poacc/billing/3.0/rules/ubl-peppol/PEPPOL-EN16931-R051/
- Explanation last updated: 2026-09-24

## The short answer

`PEPPOL-EN16931-R051` fails for each amount whose `currencyID` is not exactly the value of `cbc:DocumentCurrencyCode`. The finding location is the amount; change its `currencyID` to the invoice currency, and make sure the figure really is in that currency.

There is one exception. When `cbc:TaxCurrencyCode` names a VAT accounting currency, the VAT total in that currency goes in a second `cac:TaxTotal` with no breakdown, and that amount keeps its own currency.

## What the rule checks

The rule visits every amount element of the kinds used in an invoice, including `cbc:PriceAmount` and the amounts of allowances and charges at every level, and compares its `currencyID` with the text of `cbc:DocumentCurrencyCode` under the root. The comparison is exact, so case and spaces matter.

The `cbc:TaxAmount` of a `cac:TaxTotal` that has no `cac:TaxSubtotal` is skipped; that is where the accounting-currency VAT total belongs. When tried, a GBP invoice with a `cbc:TaxCurrencyCode` of `EUR` and a second tax total of 13.46 EUR validated cleanly.

The exception follows the shape of the tax total, not the declared currency. When tried, a second tax total in EUR that repeated the VAT breakdown was reported by this rule on all three of its amounts, together with `PEPPOL-EN16931-R053` and several VAT breakdown rules.

When `cbc:DocumentCurrencyCode` is missing or empty there is nothing to match, and every amount in the document is reported. The missing code itself is `BR-05`.

A code that is not on the ISO 4217 list fails this rule too, since it cannot equal a valid document currency; the code list rules `BR-CL-03` and `PEPPOL-EN16931-CL007` then appear with it.

| Term | Meaning | UBL element |
|---|---|---|
| BT-5 | Invoice currency code | `cbc:DocumentCurrencyCode` |
| - | Currency of an amount (the currencyID attribute) | `currencyID on each amount element, for example cac:InvoiceLine/cac:Price/cbc:PriceAmount/@currencyID` |
| BT-6 | VAT accounting currency code | `cbc:TaxCurrencyCode` |
| BT-111 | Invoice total VAT amount in accounting currency (exempt) | `cac:TaxTotal/cbc:TaxAmount, in the cac:TaxTotal that has no cac:TaxSubtotal` |

## How an integration ends up here

Possible causes, from the shape of the rule rather than from measured usage:

- Prices come from a price list in a different currency from the invoice, and the price element takes the list currency while the totals use the invoice currency.
- The invoice is converted for the buyer, and some amounts are converted and relabelled while others are not.
- The document currency is set per customer on the header, while the `currencyID` attributes are hard-coded in the template.
- VAT in a local currency is sent as a full copy of the breakdown instead of a single accounting-currency amount.

## How to fix it

1. Decide the invoice currency and write it to `cbc:DocumentCurrencyCode`.
2. Express every amount in that currency. If a price comes from a list in another currency, convert it before the line is built; changing only the attribute would leave the wrong figure.
3. Fill every `currencyID` from the document currency in one place in your mapping.
4. If VAT must also be stated in a local accounting currency, add `cbc:TaxCurrencyCode` and a second `cac:TaxTotal` holding only a `cbc:TaxAmount` in that currency, with no `cac:TaxSubtotal`.

## Before and after

These are fragments, not complete documents. The complete synthetic documents they come from are linked below.

Fragment of the failing invoice: a GBP invoice with its item price in EUR

```xml
<cbc:DocumentCurrencyCode>GBP</cbc:DocumentCurrencyCode>
<!-- parties, tax total and monetary totals omitted from this fragment -->

<cac:InvoiceLine>
  <cbc:ID>1</cbc:ID>
  <cbc:InvoicedQuantity unitCode="C62">2</cbc:InvoicedQuantity>
  <cbc:LineExtensionAmount currencyID="GBP">25.00</cbc:LineExtensionAmount>
  <!-- item omitted from this fragment -->
  <cac:Price>
    <cbc:PriceAmount currencyID="EUR">12.5</cbc:PriceAmount>
    <cbc:BaseQuantity unitCode="C62">1</cbc:BaseQuantity>
  </cac:Price>
</cac:InvoiceLine>
```

Fragment of the corrected invoice: the item price is in GBP, the invoice currency

```xml
<cac:Price>
  <cbc:PriceAmount currencyID="GBP">12.5</cbc:PriceAmount>
  <cbc:BaseQuantity unitCode="C62">1</cbc:BaseQuantity>
</cac:Price>
```

The only difference is the `currencyID` on the item price: `EUR` in the failing invoice and `GBP` in the corrected one, on an invoice whose currency code is `GBP`. `EUR` is a valid ISO 4217 code, so the code list rules stay quiet and the EN 16931 layer passes, leaving this Peppol rule as the only finding. In the recorded example a relabel is the right fix because the price of 12.5 was always meant in pounds. A price that really was in euros would need converting as well.

### What the validator reported

- The failing invoice reports **PEPPOL-EN16931-R051**. The corrected document passes every layer with no findings.
  - [Download the failing XML](https://ironfang.uk/finance/rule-examples/PEPPOL-EN16931-R051-invalid.xml)
  - [Download the corrected XML](https://ironfang.uk/finance/rule-examples/invoice-minimal.xml)

Recorded on phive 12.1.0 / phive-rules-peppol 4.5.6 / Saxon-HE 12.10, the engine behind the free validator, using synthetic data. A recorded result is regression evidence for these documents; it is not a certification.

## Where it applies

- Applies to UBL `Invoice` and `CreditNote` alike. When tried, a credit note with an `EUR` price on a `GBP` document reported this rule alone.
- It is reported by the Peppol layer. In the recorded example the EN 16931 layer passes, because none of its rules compares the currency of an item price with the document currency.
- A wrong currency on the VAT total with the breakdown matters to EN 16931 too. When tried, `EUR` there also brought `BR-CO-15`, which then finds no VAT total in the document currency.
- A second tax total in another currency, sent without `cbc:TaxCurrencyCode`, is not reported here: when tried, `PEPPOL-EN16931-R054` reported it instead.

## Related rules

- [BR-05 requires the document currency code that every amount is compared with](https://ironfang.uk/docs/finance/rules/BR-05.md)
- [BR-CL-03 checks that each currencyID is a valid ISO 4217 code](https://ironfang.uk/docs/finance/rules/BR-CL-03.md)
- [PEPPOL-EN16931-R054 governs the second tax total that carries VAT in the accounting currency](https://ironfang.uk/docs/finance/rules/PEPPOL-EN16931-R054.md)
- [BR-CO-15 needs exactly one VAT total in the document currency](https://ironfang.uk/docs/finance/rules/BR-CO-15.md)

## Scope and source

Written for Peppol BIS Billing 3.0.21 (May 2026), EN 16931 1.3.16, as applied to UBL 2.1 Invoice and CreditNote documents. Other profiles, syntaxes and releases can define this identifier differently. Guidance version 2026-09-24.1: source checked 2026-09-24, explanation last updated 2026-09-24.

[The official definition of PEPPOL-EN16931-R051](https://docs.peppol.eu/poacc/billing/3.0/rules/ubl-peppol/PEPPOL-EN16931-R051/) carries the normative wording and test. This page is our explanation of it, not a copy.

Guidance does not change the engine verdict. Fixing this finding does not mean the document passes every layer, and validation does not certify legal or tax compliance or transmit a document over Peppol.

## Links

- [This rule as a web page](https://ironfang.uk/docs/finance/rules/PEPPOL-EN16931-R051)
- [Free Peppol invoice validator](https://ironfang.uk/tools/peppol-validator)
- [Rule index](https://ironfang.uk/docs/finance/rules.md)
- [Ironfang Finance API docs](https://ironfang.uk/docs/finance)
- The same rule is available to MCP clients as the tool `finance.rule.get` on https://mcp.ironfang.uk/mcp