A customer disputes an invoice from three years ago. What can you prove?

With a disputed invoice four questions come into play, and each question calls for its own evidence. Is the invoice unchanged since it was sent? Did you really send it? Did your customer receive it? And did the supply take place? After three years nobody remembers that off the top of their head. What you have then is what you recorded at the time. This article walks through the objections a customer can raise, and for each one names the evidence that answers it.

What the law already asks of you

You must still have an invoice that is three years old. In the Netherlands you keep invoices for 7 years, in Germany for 8 years and in France for 6 years for tax purposes. See the retention periods per country.

Keeping them is not enough. The EU VAT Directive (2006/112/EC, article 233) requires that “the authenticity of the origin, the integrity of the content and the legibility of an invoice” are ensured from the moment of issue until the end of the retention period. How you do that is up to you. As a means, the directive names “any business controls which create a reliable audit trail between an invoice and a supply of goods or services”.

That audit trail is exactly what you need in a dispute.

Four questions, four kinds of evidence

Each piece of evidence answers one question. Anyone who mixes them up thinks they can prove more than they have.

What you want to show	With what	What it does not show
The invoice has not changed since a given moment	a hash, recorded at that moment, with a timestamp from a third party	that the customer received it, or that the supply was right
You sent the invoice	a record of the moment, the address and the file	that the customer received or read it
Your customer received the invoice	something from the customer’s side: a reply, a payment, an acknowledgement of receipt	exactly what was in the file
The supply took place	quote, work order, packing slip or photos	that the invoice has been unchanged since

”I never received that invoice”

Here two questions get mixed up: did you send the invoice, and did your customer receive it? Those are two different things.

For the sending, what counts is: which invoice, to which address, at what moment. An email in your sent folder is weak evidence for that. You can edit that folder yourself, and the attachment is not demonstrably the same as the file in your records. A record outside your own mailbox is stronger, with the time of sending, the address and a reference to exactly the file that was sent.

A record like that proves the sending, not the receipt. Whether your customer got the invoice is shown by something on their side: a reply to the email, a payment or an acknowledgement of receipt. If this objection comes up often, look at Peppol. There the invoice goes straight into your customer’s accounting system, not to a mailbox.

”This is not the invoice we received”

Now it is about the integrity of the content: is this file still exactly what you sent at the time?

The modification date of a PDF does not prove that, and neither does a PDF that conforms to PDF/A. A file can be edited and saved again. What does work:

A fingerprint. A cryptographic hash, such as SHA-256, is a short string of characters calculated from the content of the file. Change one byte and the whole hash changes. A hash only tells you whether two files are the same. It does not tell you when a file came into being.
A record you cannot rewrite yourself. The moment comes from the place where you record the hash. In a hash chain, every entry contains the hash of the previous one. Anyone who alters one old entry breaks the chain from that point on. Record the hash like this at the moment of sending, and later you can show that the file has stayed the same since. See how a hash chain works.
A timestamp from a third party. With an RFC 3161 timestamp, an independent service declares that a hash already existed at a given moment. That binds the moment to the hash. The European eIDAS Regulation (article 41, paragraph 1) provides that an electronic timestamp may not be refused as evidence in legal proceedings solely because it is electronic. How much weight it carries depends on the type of timestamp and on the circumstances.

The guide how to prove a PDF has not been altered goes into this in more depth.

With an e-invoice, one more thing counts. The structured XML is the actual invoice, and you keep it in the format in which you sent it. In a hybrid invoice, such as Factur-X, that XML sits inside the PDF itself. Then you keep one file with both inside.

”We never ordered this”

This objection is not about the file but about the supply. Here the audit trail that article 233 speaks of helps: the documents that connect the invoice to what you actually did.

The quote or the order confirmation, with the price and the agreement.
The work order or the completion report, preferably signed by the customer.
Photos of the result, for work on site.
The packing slip or the proof of delivery, for goods.

Send those documents along with the invoice, in the same email. Then the evidence belongs to the invoice from day one, and does not sit somewhere else in a folder. Read why those documents belong next to the invoice and not inside it.

”This invoice is wrong”

Sometimes the objection is about the content: a wrong VAT rate, a missing VAT number, a calculation error. You prevent that best before sending, with a check on the mandatory details and the calculation rules of the European standard EN 16931. Rule BR-CO-10, for example, requires the sum of the line amounts to equal the total of the lines.

If an invoice has already been sent, do not quietly change it. Send a credit note and, if needed, a new invoice. Then the trail stays complete: the old invoice, the correction and the new invoice, each with its own number and date.

What to set up today, so you have it in three years

Keep every invoice in the format in which you sent it, so for an e-invoice the XML.
When sending, record which file went to which address, and when.
Record a hash of every file you send, in a record that you cannot change afterwards without it being noticed.
Send the quote, the work order or the photos along with the invoice.
Correct with a credit note, never by overwriting an invoice you sent.
Check every invoice before it goes out.

How SealDoc records it

When it creates and sends an invoice, SealDoc records the following:

SealDoc checks every invoice against the schema and the rules of the chosen format before it is ready, and every PDF against PDF/A-3.
SealDoc puts the e-invoice as XML inside the PDF/A-3, and also delivers the XML separately. That way you keep the invoice in the format in which you sent it.
SealDoc records a fingerprint of every document, as SHA-256 and SHA-384, in a log in which every entry is tied to the one before. That way you can show later that the file has not changed since.
SealDoc emails the invoice with your attachments and records which invoice, with which attachments, went to which address and when. That is your proof of sending.
SealDoc creates the credit note directly from the original invoice.

Need to hand the evidence to an accountant or a lawyer? On the Platform packages, from Starter, you download an evidence pack per document: one ZIP with the document, the log and a manifest with the hash of every file. Read what is in an evidence pack.

Frequently asked questions

How long must I be able to prove that an invoice is genuine?

For the whole retention period. Article 233 of the VAT Directive requires that the authenticity of the origin, the integrity of the content and the legibility are ensured from the moment of issue until the end of the retention period. For invoices that is 7 years in the Netherlands, 8 years in Germany and 6 years in France.

Is a sent email proof that my customer received the invoice?

A sent email shows that you sent it, not that it was read. So record the time of sending, the address and the file you sent, outside your own mailbox.

Does PDF/A prove that an invoice has not changed?

No. PDF/A is about legibility in the long term. A PDF/A can be edited and saved again as a valid PDF/A. You prove integrity with a hash that you record at the moment of sending.

May I change an invoice I have sent?

Do not change a sent invoice. Correct it with a credit note and, if needed, a new invoice, so that the original invoice and the correction both stay in your records.

In short

What counts in a dispute over an old invoice is what you recorded at the time. Four questions, each with its own evidence: is the invoice unchanged, was it sent, was it received, and did the supply take place? Record what you can at the moment the invoice goes out, not when the customer calls. Start with invoicing in SealDoc, or see how SealDoc records evidence.