Invoice numbers: one unique series per entity

In short

Every invoice number must point to one document from one seller, and never to any other. Give each company its own series, give credit notes a series of their own, and add the year where numbers restart. Set this up in your system once, before go-live.

Why eInvoicing needs it

The VAT rules already require a tax invoice to carry a sequential number, or a unique number that identifies the invoice and its place in your series. The Ministry of Finance list of mandatory eInvoice fields describes the invoice number as "a unique identification of the Invoice".

With eInvoicing, your invoice number travels as data to your customer and to the Federal Tax Authority (FTA), alongside a separate unique ID your eInvoicing service provider creates. Your own number still matters: it is what your customer pays against, what your credit notes refer to, and what the FTA sees when it compares documents. Two documents with the same number from the same seller look like a duplicate.

The mistakes Calculus staff see most often:

• Two companies in one group print the same number on different invoices.
• Voucher numbers restart each year, so "0001" exists once for every year.
• Credit notes and invoices share one series.
• Dates typed as text ("April 29th", "29.04.26") instead of true dates.

What good looks like

An invented group with two companies, Al Noor Trading LLC (AN) and Gulf Spice Imports FZE (GS):

Document	Before	After
Al Noor invoice	151	AN-2026-0151
Gulf Spice invoice	151	GS-2026-0151
Al Noor credit note	152 (same series as invoices)	AN-CN-2026-0007
Invoice date	"29.04.26" typed in a text cell	2026-04-29, a true date

Every number is now unique across both companies, every year and every document type. The date travels as year, month and day, so no system reads it the wrong way.

How to fix it in Tally

1. Press Alt+G (Go To), choose Alter Master, then Voucher Type, and select your sales voucher type.
2. Set Method of Voucher Numbering to Automatic, or Automatic (Manual Override) if you must sometimes type a number.
3. If you use Automatic (Manual Override) or Manual, set Prevent creating duplicate Voucher Nos to Yes. Tally allows this only on a voucher type with no entries yet, so you may need a new voucher type from your start date.
4. Set Set/Alter additional numbering details to Yes. In the next screen, enter an Applicable From date and the Prefix Details, for example AN-2026-. Set a width and Prefill with zero if you want 0001.
5. If numbers restart each year, set the restart Applicable From date, Starting Number and Periodicity, and add a new prefix with the new year from the same date.
6. Repeat for the credit note voucher type with its own prefix, for example AN-CN-2026-.
7. In a second company, use a different prefix.

How to fix it in Excel or another system

• Keep an invoice register sheet: one row per invoice, with the number, the true date and the customer. Take the next number from the register, never from memory.
• Build the number from parts: ="AN-"&YEAR(C5)&"-"&TEXT(D5,"0000") gives AN-2026-0151.
• Format the date column as a date, not as text. Example value: 2026-04-29.
• Use a separate register for credit notes.

On an ERP, ask your ERP partner to set one numbering series per legal entity and document type, with a prefix, and to block duplicates.

Checklist

• Each company has its own invoice series with its own prefix.
• Credit notes have their own series.
• Numbers that restart each year carry the year.
• Duplicate numbers are blocked in the system.
• Invoice dates are true dates, never typed text.
• The start date for the new series is agreed and written down.

The law

• VAT Executive Regulation (Cabinet Decision No. 52 of 2017, as amended), Article 59(1)(d): a sequential tax invoice number, or a unique number that identifies the invoice and its order in any sequence of invoices; Article 59(1)(e): the date of issue. https://mof.gov.ae/wp-content/uploads/2025/10/Cabinet-Decision-No.-52-of-2017-of-the-Executive-Regulation-of-the-Federal-Decree-Law-No.-8-of-2017-on-Value-Added-Tax-and-its-amendments.pdf
• VAT Law (Federal Decree-Law No. 8 of 2017, as amended), Article 65: the registrant issues a tax invoice, with its contents set by the Executive Regulation. https://uaelegislation.gov.ae/en/legislations/1227/download
• MoF UAE Electronic Invoice mandatory fields V1.0, field 1 Invoice number ("a unique identification of the Invoice") and field 2 Invoice date. https://mof.gov.ae/wp-content/uploads/2026/02/UAE-Electronic-Invoice-mandatory-fields_V-1.0-23Feb2026.pdf
• MoF UAE Electronic Invoicing Guidelines V1.1, section 3: the service provider's unique ID is created in addition to the invoice sequential number. https://mof.gov.ae/wp-content/uploads/2026/06/UAE-Electronic-Invoicing-Guidelines_V-1.1-01June2026.pdf
• Peppol PINT AE, Invoice number (IBT-001) and Invoice issue date (IBT-002). https://docs.peppol.eu/poac/ae/pint-ae/