MosoFin skill · customer-invoicing · learning guide

From one invoice to a clean number sequence

This page shows how the skill prepares an invoice and checks the invoices you already sent, one level at a time. It starts with the fields that make an invoice valid. It ends with the things the skill never does. The page quotes every rule from the skill and works every number in front of you.

The rules are the skill's ownThe page quotes each rule word for word, with the file and section it comes from.

Outside sources back the factsThey are three GOV.UK and HMRC pages, a European Commission page and a Federal Reserve survey. Each one has a number and a link.

The page shows the mathOpen "Check it yourself" on any level. Follow every line on a calculator.

Your numbers, your checkAt the end, enter up to 8 invoice lines and a list of invoice numbers. The page checks the totals and the sequence.

Rule: quoted from the skill Fact: from a cited authority, numbered like ¹ Example: made-up figures, calculated in full

How it thinks

Eight rules the whole skill runs on

Every level below applies one or more of these rules. The examples use one made-up company, Northbrook Trading Ltd. It has a UK VAT registration and keeps its books in pounds. The owner says that UK VAT governs its sales. The UK is one example jurisdiction. The skill works with the rules of any jurisdiction that you give it. Northbrook numbers its invoices in one series, NT-YYYY-NNNN.

Learning path

Six levels, from simple to complex

Check it yourself

Check your own invoice

The calculator has two parts. Part 1 checks the lines and totals of one invoice. Part 2 checks a list of invoice numbers for gaps and duplicates. The page sends nothing anywhere.

Part 1: the lines and the totals

Description	Qty	Unit price £	VAT rate %	Standard price £	Last price to this customer £

The page rounds the VAT on each line to the penny. It also calculates the VAT on the subtotal of each rate and rounds it once. The rounding difference is the sum of the line VAT minus the VAT on the subtotals. The skill uses the rounding method that you state, and reports a difference of more than £0.01 between that method and the invoice. The page flags a price that differs from the standard price and from the last price to this customer. If you leave one of those two prices blank, the page compares with the other one only. The £250 limit is a UK rule ¹. Other jurisdictions differ.

Part 2: the number sequence

Invoice numbers, one for each line or with commas between them

The page splits each number into a prefix and a final run of digits. It checks each prefix as one series, from its lowest number to its highest. Paste every number in the range, voided invoices included. A voided invoice still appears in the list, at zero. So a number that is missing points to a deleted invoice. Only the audit log of your accounting system can prove a deletion.

Using it

What it asks, needs and gives back

Asks firstDo you want to use Mosofin for this run, or upload the data yourself? Do you want to prepare new invoices or check issued ones? Which company sells? Who is the customer, and what do you bill on each line? What are the invoice date, the tax point and the delivery date of each line? Which jurisdiction governs the supply, and which fields and phrases must an invoice show there? Which tax rate applies to each line? Is there an exemption or a reverse charge? Do your prices include tax, and how do you round the tax? What is your numbering scheme? What are the terms, the currency, the purchase order number and the payment instructions?

NeedsTo prepare, the skill needs the customer's details, the lines to bill, your legal name and your tax registration. It also needs your numbering scheme with a list of recent invoices, and the chart of accounts. To check, it needs the issued invoices and an invoice list for the full range, voided invoices included.

Gives backThe skill gives back an invoice draft, marked for review, and the matching journal entry as a proposal. The entry must agree with the amount due. In check mode, it gives back each missing field, each arithmetic error, each duplicate number and each gap.

NeverThe skill never uses Mosofin before you agree. It reads nothing before you answer its questions and say yes to its summary. It never creates an invoice, gives out a number, sends anything or posts an entry. It never assumes the jurisdiction, a tax rate, the invoice date or the numbering scheme. It reads the next number again on every run. It never saves tax IDs, bank details or invoice contents.

Sources

Where every fact on this page comes from

The rules are the skill's own. The facts below come from five outside sources. Sources 1, 3 and 5 are UK sources. Source 2 is an EU source. Source 4 is a US source. MosoFin checked each one on 27 October 2026 from search results and did not open the pages. Follow the link for today's wording.

Not backed by an outside authority. The six stages, the prepare and check modes, and the price check against the standard price and the customer's history are the skill's own. MosoFin made up Northbrook Trading, Harbor Retail, the invoices and every figure in the examples. The page marks them where they appear.