Which currency, and which exchange rate, a Greek invoice has to use.

The provision

Greece puts the currency rule in its accounting standards law.

The Greek invoice particulars sit in the law on accounting standards, not the VAT Code. Its article 9 lets the amounts be in any currency and requires the invoice's VAT amount to be expressed in the national currency as well.

«το ποσό Φ.Π.Α. του τιμολογίου εκφράζεται και στο εθνικό νόμισμα»

the VAT amount of the invoice is also expressed in the national currency

Art. 9 par. 2, N. 4308/2014 (FEK A' 251/2014)

When it reaches you

If you sell through the stores, this bites in one case.

Article 230 constrains the amount of VAT payable or to be adjusted. An app developer's ordinary invoice to a store or an ad network carries no VAT amount at all: it is a reverse-charge supply to another member state, or a supply outside the scope entirely. Where there is no tax line, there is nothing for the currency rule to bind. The rule reaches you on a supply that actually charges Greece VAT.

What Ureta does about it

The rate is pinned when the month is, and it is kept with the document.

• The ECB reference rate, for every EU tenant. It is the rate article 91(2) obliges every member state to accept. A currency other than the euro is converted through the euro, as the same article says.
• The rate is pinned per month and stored, so a document issued last March still reports the rate it carried in March rather than a number recomputed today.
• Nothing is guessed. Where no rate from a defensible authority is available for a currency, the engine refuses to issue rather than putting a plausible wrong number on a legal document.