A Thursday afternoon email from your Polish client: we are not paying this, the VAT rate is wrong. The instinct is to pull the document and reissue it. In KSeF (Poland's national e-invoicing system) that door is closed. Once the system assigns an identification number, the invoice is issued and it stays.
That changes how a dispute gets handled. The question is no longer whether something can be undone, but how fast you issue the correct document and how you explain it. Since 1 February 2026 every error on an invoice, including purely formal ones, is the seller's responsibility alone.
Here is how to triage the dispute, pick the right correction, know which VAT period it lands in, and write the message that keeps the relationship intact.
Start by asking what exactly is being disputed
Clients rarely say "this needs a value correction." They say the invoice is wrong. Your first job is to sort the complaint into one of three buckets, because each has a different path.
What the client disputes Type of correction Amounts shown Formal data: company name, address, payment term, delivery date Correction of formal data No amounts, you state the correct content of the affected fields Price, quantity, VAT rate, a missing discount Value correction, in minus or in plus The difference, split by VAT rate Invoice issued to the wrong legal entity (wrong NIP) Correction to zero plus a new invoice All lines zeroed
There is a fourth case that looks alarming and is not a problem at all: an invoice rejected by KSeF. A rejected document is not considered issued, so there is nothing to correct. You fix the file and send it again. The Ministry of Finance confirms this in its KSeF questions and answers.
Two things you can no longer do
Cancel the invoice. A document accepted by KSeF and stamped with an identification number cannot be deleted or edited. The record is permanent.
Rely on a correction note. The provision that let the buyer fix descriptive fields on their own side was repealed. Your client has no tool to clean up your typo. Every change, from an amount down to a street address, requires a correction invoice from you.
The practical consequence for the relationship is straightforward: your response time is now part of the service. A buyer who used to close the matter in five minutes with their own note is now waiting on you. If they wait three days, they remember it at renewal.
The correction itself must be structured and must carry the KSeF number of the invoice being corrected, under Article 106j(2) of the VAT Act. The exception covers invoices that never received a KSeF number.
Choosing the correction type
The FA(3) schema, mandatory since 1 February 2026, requires you to declare why you are correcting. This is not a throwaway field, because the reason drives the VAT period for in-plus corrections.
Error. The cause existed when the original was issued: wrong VAT rate, ten units billed instead of one. In-plus corrections go back to the original period.
New event. The cause arose later: goods returned, a post-transaction rebate, a change in service scope. In-plus corrections are booked in the period the cause arose.
Mixed. One document covers both.
In a dispute this classification is also a conversation tool. "We applied a rebate that was not on the order" reads very differently from "we used the wrong rate." The first is a new event, the second is an error, and your client's accountant will see the difference on the document.
When the in-minus correction hits your VAT return
This is where KSeF genuinely helps, and it is worth knowing because it is an argument you can use.
An in-minus correction issued online is settled in the period you issued it in KSeF, and a structured invoice counts as issued on the day it was sent to the system. You no longer need documentation of an agreement with the buyer or proof that they received the correction. Acceptance by the system serves that role (Article 29a(13) and (14) of the VAT Act).
If the correction was created in offline24 mode, during a declared system unavailability, or during an outage, and then delivered to the buyer through KSeF, you reduce the base in the period you sent the correction to the system (Article 29a(13c)).
Your client faces the mirror rule: the buyer reduces input VAT in the period they received the correction (Article 86(19a)).
Rules for in-plus corrections did not change and are set out in Article 29a(17).
One technical trap worth flagging for teams running high volumes: if the original invoice was created in offline24 mode and does not yet have a KSeF number, send the original to the system first, then issue the correction. The Ministry of Finance describes this in its section on offline24 mode.
How this works in Biurko
The path starts from the invoice view, not from a blank form. You choose "issue correction" and the system builds a draft linked to the source document: it carries over the lines together with their pre-correction values, copies the party details as they appeared on the original, and fills in the KSeF number of the invoice being corrected.
A few decisions we made deliberately:
Reason and correction type are mandatory. A correction with an empty reason will not go out. The KSeF adapter raises an error rather than shipping a blank field to the system.
Correction to zero is a separate action. One click zeroes every line and every total. That is the path for a wrong buyer NIP, after which you issue a fresh invoice to the correct entity.
One draft correction per invoice. A double click during a stressful phone call returns the existing draft instead of creating two documents.
Currency mismatch is blocked. A correction must use the same currency as the original, because a difference amount across two currencies is undefined. Better to stop it locally than to collect a rejection from KSeF.
Full set of corrective types: KOR for a VAT invoice, KOR_ZAL for an advance invoice, KOR_ROZ for a settlement invoice, and KOR_RR for a flat-rate farmer invoice under the FA_RR schema.
Document history. The log shows who created the correction, when, and which document numbers are linked. When a client reopens the matter a month later, you are not reconstructing it from memory.
To be straight about a limit: the FA(3) schema allows one correction to reference several original invoices, but the standard path in Biurko is one to one. If you need a collective correction covering a dozen invoices for one counterparty, talk to us before you plan around it.
What to write to the client
The technical part takes a few minutes. The relationship is decided by the message you send alongside it.
Three rules that work:
Acknowledge the same day, even if the correction goes out tomorrow.
Name specifically what you are fixing, and skip the three-paragraph explanation.
Include the KSeF number of the correction so their accounting team can find it immediately.
An example:
Thanks for flagging this. On invoice FV/2026/03/118 we applied 23% VAT instead of 8%. We have issued correction FK/2026/03/12, KSeF number: [number]. It is already in your KSeF inbox. The remaining amount due is X PLN, payment term unchanged. Apologies for the trouble.
What to avoid: "we cannot undo it, that is how the system works." The client hears "your problem." The same fact framed correctly does the job: "invoices in KSeF cannot be cancelled, so we issued a correction and it is already in the system."
Checklist for a disputed invoice
Decide whether the dispute is about formal data, amounts, or the buyer's identity.
Check whether the original has a KSeF number. Without it the correction has nothing to reference.
Pick the correction type: error, new event, or mixed.
Write a reason the client's accountant will understand, not just you.
For a wrong NIP, issue a correction to zero and a separate new invoice.
Send the correction to KSeF and note its number.
Message the client with the correction number and the remaining amount due.
Wrapping up
KSeF has no undo. What it does have is a single, uniform way to fix an error and, for the first time in years, in-minus settlement without chasing counterparties for confirmations. The difficulty moved from paperwork to response time and communication.
Biurko handles the technical half: it links the correction to the source document, enforces reason, type and currency, supports KOR, KOR_ZAL, KOR_ROZ and KOR_RR, and sends everything to KSeF using certificate-based authentication. The conversation with the client is yours.
Create a free account at biurko.io and issue your first correction with no time limit.
FAQ
Can you cancel an invoice in KSeF? No. A document that received an identification number is treated as issued and cannot be deleted or edited. A correction invoice is the only route. The exception is an invoice rejected by KSeF, which was never issued in the first place.
Can the buyer issue a correction note for a KSeF invoice? No. The provision covering correction notes was repealed on 1 February 2026. All errors, including formal ones, are fixed by the seller through a correction invoice in KSeF.
When do I settle an in-minus correction issued in KSeF? In the settlement period in which you issued the correction in KSeF, meaning the period you sent it to the system. No confirmation of receipt and no documentation of agreed terms is required.
What if the invoice went to the wrong NIP? Issue a correction to zero addressed to the incorrect buyer, then issue a new invoice to the correct entity. A data correction is not enough, because the party to the transaction changes.
How many corrections can be issued to one invoice? The regulations set no limit. Each correction must reference the KSeF number of the original invoice, so the chain of documents stays linked.