Debit note: how to increase the taxable amount of an invoice you already issued

What a debit note is

A debit note (in Bulgarian, дебитно известие) is a primary accounting document issued against an invoice that already exists, whenever the taxable amount of a supply needs to go up after the invoice has been issued. It does not replace the invoice and is never issued on its own: it always references one specific, already-issued invoice and adjusts it upward.

The taxable amount is the value of a supply on which VAT (Bulgaria's value-added tax, ДДС) is charged. When that value changes after invoicing, Bulgarian law requires the change to go through a separate note, not through editing the original invoice and not through issuing a brand-new invoice for the difference.

A debit note is the mirror image of a credit note. A credit note lowers the taxable amount (for a discount, a returned good, or correcting an error downward), while a debit note always increases it. The National Revenue Agency (in Bulgarian, НАП, commonly abbreviated NRA) is the authority that checks how companies report these notes in their VAT filings.

When you need to issue a debit note

You issue a debit note whenever supplier and client agree on something that raises the value of an already-invoiced supply. Four situations come up most often in practice.

• Additional charges after the invoice: the client orders extra work or an add-on service related to a supply that was already completed and invoiced, and both sides agree the amount should attach to that same deal rather than a new one.
• A price increase agreed after invoicing: for example, a contract that recalculates the price based on an exchange rate, an index, or a volume threshold, and the recalculation comes out higher than what was originally charged.
• An undercharged amount from the original invoice: a calculation or clerical error left the invoiced value lower than what was actually owed, and the fix goes through a note rather than a fresh invoice.
• Extra quantities delivered later: the supplier ships more goods or delivers more service hours than the original supply covered, and both sides agree the difference should attach to the existing invoice instead of becoming a separate sale.

In every one of these cases, a debit note is the legal way to raise the value of something that has already gone through an invoice, without unwinding the original document.

Mandatory details: the reference to the original invoice is not optional

A debit note carries the same general mandatory details as an invoice: supplier and client names and addresses, the issue date, a description of the supply, the value, and the applicable VAT rate. On top of that, a debit note has a few details unique to it.

• The number and date of the original invoice the note is issued against. This is a mandatory element: a note without a clear reference to the specific invoice it corrects is not a valid document under Bulgarian law.
• The reason for increasing the taxable amount: a short description of why the value is being adjusted upward, such as an additional supply, a contractually agreed price increase, or a correction of an undercharge.
• The size of the increase, stated separately as the increase in the taxable amount and the corresponding increase in VAT due.

In practice, this means you need the number and date of the original invoice on hand before you can issue the note. If you invoice electronically, the software typically fills in that link automatically, which cuts the risk of a mismatch showing up later during a review.

Numbering: notes are not a separate series

Debit notes are not numbered in their own, separate sequence. They share the same consecutive numbering series as the company's invoices: if the last invoice issued was number 145, the next document, whether it is another invoice or a debit note, continues that same series from 146.

This has a practical consequence during a review: an NRA auditor expects the numbers of every issued document, invoices and notes together, to run without gaps or duplicates. A company that keeps a separate numbering series just for notes is flagging an inconsistency that invites extra questions during an audit.

If you issue documents manually, or through a system that does not track the combined sequence automatically, it is good practice to keep a single register for all outgoing invoices and notes, sorted chronologically by number rather than by document type.

The VAT effect and the 5-day deadline

A debit note has a direct effect on both parties' VAT position. The supplier charges the additional VAT on the increase in the taxable amount and reports it in the sales ledger for the period in which the note is issued. If the client is also VAT-registered and entitled to deduct input VAT, they record the same increase in their own purchase ledger and deduct the additional VAT under the usual rules.

The law sets a clear deadline: the note must be issued no later than 5 days from the date the circumstance triggering the increase occurred, for example the date both sides agreed on the additional amount, or the date the error was discovered. Missing the deadline does not cancel the obligation, but it is grounds for a finding during a tax audit and potentially for interest charges.

This is worth repeating alongside the credit note comparison, because mixing the two up is a common mistake for small companies: a credit note reduces an already-charged taxable amount and the related VAT, while a debit note always increases both. The direction of the correction, down or up, is the only difference between the two documents, but it decides which one you need to issue.

A debit note is part of the invoice's paper trail

A debit note rarely stands alone: it is one piece of a wider set of primary accounting documents that travel alongside every invoice, from credit notes for the opposite correction to the fuller documentation required for cross-border supplies. The sooner that paper trail is in order, the fewer questions you will face at the next tax review.

Useful next reads:

How invoicing will work with FreeMe.bg
How VAT and tax filing will work with FreeMe.bg
Tracking your company's expenses with FreeMe.bg
How to issue an invoice in Bulgaria: the full guide

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