Invoice currency

An invoice may be in any currency, and its tax is still owed in the currency of the place that levies it. A Danish sale invoiced in euros owes Danish kroner, and Art. 91 of the VAT Directive says how to convert: at the rate in force when the tax became chargeable, which a member state may set as the European Central Bank's.

This applies inside the Union. Outside it, the assessment carries no rate: where another country asks for its tax in its own currency, it names its own rate, and that conversion is yours.

What an assessment carries

When the amounts are in another currency than the place of supply's, the assessment carries the rate for its tax point, and converts on demand:

$assessment->tax;                       // EUR 25.00
$assessment->exchangeRate;              // EUR → DKK 7.463, ECB, 2026-09-25
$assessment->taxInLocalCurrency();      // DKK 186.58

$order->taxInLocalCurrency();           // the document's tax, converted once from the total

The order's figure is converted once, from the total. Each line converted and rounded on its own can drift from it: three lines of 0.01 EUR are 0.07 DKK each, 0.21 together, where the order's 0.03 EUR is 0.22 DKK. Print the document's figure on the invoice.

Where amounts and place share a currency, exchangeRate is null and nothing is converted. It is null too outside the Union — Art. 91 is EU law, and the ECB's rate is no other country's rule — and where the invoice states no tax: a reverse charge, an exempt or zero-rated sale, or one this seller does not collect. An order converts the lines that state tax and passes over the ones that do not. Where no rate is known for the date, it is null too: nothing is refused and nothing is guessed, and the conversion is yours.

Where the rate comes from

The shipped ExchangeRates reads the ECB's euro reference rates from local disk:

php artisan tax:fx:sync     # schedule it daily

Nothing is fetched while pricing. The bank publishes one rate per currency per business day, as units per euro, and a rate between two other currencies is read through the euro. On a weekend or a holiday the latest earlier rate is in force; one older than a week is not used, because that is a store nobody synced rather than a holiday.

Your own source. Some countries take their own bank's rate, and a customs authority publishes monthly ones. Bind Cbox\Tax\Contracts\ExchangeRates to your implementation and every assessment uses it.