MosoFin · customer invoicing, in plain words

Why hasn't this customer paid? Is something missing from the invoice?

An invoice is a legal document. Most tax systems say what it must show before your customer can claim back the tax. If a field is missing, the customer may refuse to pay until you send a new invoice. This skill prepares new invoices with every field, and it checks the invoices that you sent.

The 20-second version

You tell it your tax rules, what to bill and your numbering scheme. It fills most fields from your books. It checks each line, calculates the totals again and finds gaps in your invoice numbers. It gives you a draft for review. It creates, numbers, sends and posts nothing. You or your bookkeeper enter the invoice.

Prepare: what a run gives you

Northbrook Trading bills Harbor Retail

Northbrook Trading is a UK company, registered for VAT. The owner tells the skill that UK VAT applies. The skill never guesses this from the company's country. The UK is one example. The skill asks every owner for their own jurisdiction and its list of invoice fields.

Line	Qty	Price	Subtotal	VAT 20%	Price check
Storage racking	10 units	£180.00	£1,800.00	£360.00	Same as the standard price, £180.00
Installation	6 hours	£65.00	£390.00	£78.00	Standard price £55.00. Harbor paid £55.00 last time. The skill asks you
Total			£2,190.00	£438.00	£2,628.00 to pay

Example figures for a made-up company. 10 × £180.00 = £1,800.00. 6 × £65.00 = £390.00. Subtotal: £1,800.00 + £390.00 = £2,190.00. VAT: £360.00 + £78.00 = £438.00, which is 20% of £2,190.00. Total: £2,190.00 + £438.00 = £2,628.00. Invoice number NT-2026-0418, invoice date 31 October 2026, payment terms Net 30.

The total is more than £250. So in the UK this must be a full VAT invoice¹. A full VAT invoice shows a unique number in one series, the names and addresses of both companies, and the seller's VAT number. It also shows the date of supply, the date of issue, a description, the quantity, the VAT rate and the VAT amount¹.

From the books, the skill fills Harbor's name, address and VAT number. It also fills Northbrook's legal name and address, the items and prices, the VAT code, the Net 30 terms and the next number, NT-2026-0418. The VAT number on file is what a person typed. So the skill asks you to check that it is current.

Northbrook delivered the racking and did the installation on 28 October. The invoice date is 31 October. The date of supply, which accountants call the tax point, can decide the tax period. The skill records both dates and asks you for the tax point when it differs from the invoice date. The skill also asks for the delivery date of each line. Both lines were complete before the invoice date, so the skill books them as sales. Work that ends after the invoice date goes to deferred revenue (money for work not yet done) instead.

The installation price is different from the standard price and from Harbor's last price. That can be a discount that you agreed, or a typing error. The skill asks before the invoice goes out, when a correction costs less.

It also shows the accounting entry that the invoice will make. Debit accounts receivable (money that customers owe you) £2,628.00. Credit product sales £1,800.00, service sales £390.00 and VAT payable £438.00. £1,800.00 + £390.00 + £438.00 = £2,628.00. The skill checks that the receivable is equal to the amount due. That is the £2,190.00 subtotal, minus a £0.00 discount, plus £0.00 shipping, plus £438.00 VAT. Most accounting systems make this entry automatically when you enter the invoice. So the skill's entry is a record of what will happen.

The mistake it catches

Check the invoices you already sent

Many small firms have a payment problem. Firms that let customers pay after delivery, for example by invoice, report slow payers more often². An incomplete invoice gives a customer one more reason to wait. In check mode, the skill applies the same rules to invoices that you already sent.

The invoice list as it looks

The fixed range NT-2026-0390 to NT-2026-0417, voided invoices included

Invoice numbers in the range28

Documents found28

Conclusion"All there"

The two counts agree. A person sees no problem.

What the skill reports

Each number and each invoice, checked

Missing numberNT-2026-0402

Number on two documentsNT-2026-0409

Count: 28 − 1 + 128

One gap and one duplicate hide behind a count that agrees. The skill gives each number.

The list includes the voided invoices, so a voided number is not a gap. A missing number usually means that a person deleted an invoice. Only the audit log of your accounting system can prove a deletion, and the skill cannot read it. In several jurisdictions, a gap in the sequence is a compliance problem. The skill found three more problems in the same two months:

Rounding, NT-2026-0396£0.026 lines of £33.33 at 20%. Northbrook rounds VAT on each line: 6 × £6.67 = £40.02. The invoice shows £40.00, the VAT on the £199.98 total

Missing phrase, NT-2026-0411Reverse chargeAn Irish business customer at 0% VAT, with no reverse charge words. Its tax office can refuse the customer's claim

Missing field, NT-2026-0414AddressNo customer address. A UK invoice must show it³

That is five findings: 1 gap, 1 duplicate, 1 rounding difference, 1 missing phrase and 1 missing field. The skill calculates the totals again with your own rounding method. It reports each difference of more than £0.01 that stays.

How a run starts

Nine questions, in one message

The skill asks these questions first, in one short batch. It asks only for what it cannot already see.

Use MosoFin for this run, or upload the data yourself?
MosoFin is the connection to your books and the other platforms you link. With it, the skill reads from there. Without it, you upload the data, and the checks are the same. The skill asks this only if your account has a MosoFin connection.
Prepare new invoices, or check invoices that you already sent?
Required. They are different jobs.
Which company is selling?
Required. Its legal name, tax registration and number series go on the invoice. An invoice from the wrong company is legally wrong.
Which customer, and what to bill: the description, quantity, unit price and tax code of each line?
Required. The books hold your standard prices. What to bill this time is your decision. The skill asks you to check that the customer's tax number is current.
The invoice date, the tax point if it is different, and the delivery date or service period of each line.
The tax point decides the tax period. The delivery date decides if the line goes to sales or to deferred revenue. The skill never chooses the invoice date for you.
Which jurisdiction controls this sale, its required fields and phrases, and any electronic invoicing rule?
Required. This decides the fields and the tax. Your place of tax registration and the place of the sale can be different. The skill never guesses this from your company's country.
The tax rate of each line, any exemption certificate or reverse charge, and do your prices include tax? Do you round tax on each line or on each rate total?
Required if tax applies. The skill can read the tax codes that you use. It cannot know if a rate is right for this sale. It never chooses a rate for you.
Your numbering scheme: prefixes, resets, and a separate series for each company.
Required. The skill measures gaps and duplicates against it, and never chooses it for you. With MosoFin, it reads the next number from the live series.
Payment terms, currency, the customer's purchase order number and the payment instructions.
The skill requires the terms and the currency. Some customers do not pay without their purchase order number. You check the payment instructions against a trusted source.

Then it reads your answers back, and waits for your yes:

"Preparing one invoice, two lines, from Northbrook Trading to Harbor Retail, read from your Mosofin connection. Jurisdiction UK VAT, standard rate on both lines. Invoice date 31 October. Next number NT-2026-0418, from the existing NT-YYYY- sequence, proposed, not reserved. Terms Net 30. Payment instructions as you confirmed them. Shall I go ahead?"

What you need

Connect once, or upload the invoice data

Connected with MosoFin

Say yes, then give it what the books cannot hold

It reads the customer's name, address, tax number and usual terms
It reads your legal name and address, your items and standard prices, and your tax codes
It reads your recent invoices, for the next number, the gap check and what this customer paid last time
In a group of companies, it asks which company sells. Each company has its own number series
You still give it the jurisdiction, what to bill, the invoice date, any exemption certificate and the payment instructions. You check the tax for each line and the numbering scheme

Uploading instead

Bring the details for the job

To prepare: the customer's details, the lines to bill, your legal name and tax registration. Type them in the chat, or a CSV or XLSX. Required
Numbering scheme and a list of recent invoices, for the next number. CSV or XLSX. Required
Chart of accounts with the sales, output tax and deferred revenue accounts. CSV or XLSX. Required
To check: the invoices that you sent, and an invoice list for the full range, voided invoices included. A sample cannot show gaps or duplicates. PDF, image, CSV or XLSX. Required
The invoice date, terms, currency, jurisdiction and tax rates, typed in the chat. The skill recommends the customer's purchase order number. Add an exemption certificate if it applies

What you get back

A draft, the checks, the entry and the sources

The invoice draftEvery required field, with a mark that says "draft for review". As a PDF, a web page or data for your system, as you prefer.

The check resultsFor each invoice: missing fields, line errors, totals that do not agree, duplicate numbers and gaps, each by number.

The proposed entryAccounts receivable, sales and tax, with the account names from your chart of accounts. It must agree with the amount due. Work not yet delivered goes to deferred revenue.

Where each field came fromEach field says if it came from your books or from you.

The number on the draft is a proposal. Until a person enters the invoice, someone else can use that number. The skill reads the next number again on every run. The bank details on the invoice need care too. The skill asks you to check them against a trusted source. Payment fraud on invoices works by a change to exactly those details.

The skill asks before it makes an Excel file. If your jurisdiction requires electronic invoicing through a government portal, the skill can prepare the data. You must send it through that portal yourself.

What it never does

Six rules that never bend

Asks before it uses MosoFin. A connected account is not permission to read it.
Reads nothing until you answer the first questions and say yes to the read-back.
Never writes, changes or deletes anything, in any system. Every entry is a proposal for a person to post.
Never says that an invoice exists. Every document is a draft for review. The number on it is a proposal until a person enters the invoice.
Never chooses the jurisdiction, a tax rate, the invoice date or the numbering scheme for you. It never guesses the jurisdiction from your company's country. It uses only the selling company's own records, and never drafts an invoice to a real customer from test data.
Never copies payment instructions from memory or an old document. You check them against a trusted source. It never saves customer tax numbers, bank details or invoice contents.

Words you'll hear

Six terms, in plain English

Full VAT invoice: The UK invoice that lets a VAT-registered customer claim back the VAT. The UK requires one above £250, or below it if the customer asks¹.
Tax point: The date of supply. It can be different from the invoice date, and it decides the tax period.
Reverse charge: In many VAT systems, a business customer abroad accounts for the VAT, and the seller charges 0%. The invoice must say so, if the rules of the jurisdiction allow it.
Sequential numbering: Invoice numbers in one series with no gaps. Every UK invoice needs a unique number³.
Accounts receivable: The money that customers owe you. Accountants write AR.
Net 30: Payment terms. The customer pays within 30 days of the invoice.

Sources

Where the facts on this page come from

The sources are an HMRC manual, a US Federal Reserve survey report and a GOV.UK guide. We found each one on 27 October 2026 in search results. We did not open the pages, so follow each link for today's wording. What the skill does comes from the skill's own files.

HMRC: VATREC5010, Details which must be shown on a full VAT invoiceA UK full VAT invoice shows a unique number in one series, the seller's name, address and VAT number, and the customer's name and address. It also shows the tax point, the date of issue, a description, the quantity, and the VAT rate and amount. It is necessary above £250, or below it if the customer asks.
Federal Reserve Banks: 2024 Report on Payments, Small Business Credit SurveyAbout four in five small firms have a problem with payments. Firms that let customers pay after delivery, for example by invoice, report slow payers more often.
GOV.UK: Invoicing and taking payment from customers: invoices, what they must includeEvery UK invoice shows a unique number, and the names and addresses of the business and the customer. It also shows a description, the dates of supply and of the invoice, the amount and the total.

No outside authority backs the six stages, the prepare and check modes, or the price check against the standard price and the customer's history. They are the skill's own rules. We made up every example figure. Sources 1 and 3 are UK sources. Source 2 is a US source.

About this skill

This page explains customer-invoicing, from the MosoFin finance skills library, MosoFin/mosofin-finance-skills on GitHub. Read the skill on GitHub → This page describes the skill in its card format. That format is on main of MosoFin/mosofin-finance-skills. PR #1 merged it on 26 October 2026, with a bookkeeping-audit fix on 27 October 2026. Want the worked examples? Learn it step by step →