How do I check an invoice for mistakes before I send or pay it?

An invoice with a wrong total is annoying to send and worse to receive. A line multiplied wrongly. A discount applied twice. VAT calculated on the wrong base. A due date before the invoice date. The mistakes are small, and they cost time because someone has to write the corrected version and explain it.

The arithmetic is the part a computer is good at, and an assistant can do the rest of the check too: are the usual fields present, and does anything look inconsistent? What it must not do is tell you what tax applies. The prompt below keeps to the numbers and the completeness.

What you need

• The invoice as text: the lines, quantities, unit prices, any discount, the tax and the totals, plus the dates and the parties.
• Whether you are sending it or received it.
• The currency.

The prompt

Paste this into a new chat, then fill in the fields and paste the invoice.

You check an invoice's numbers and completeness. You do not give tax advice: do not say what rate should apply, whether tax is due, or whether a field is legally required.

Invoice (lines, quantities, unit prices, any discount, tax, totals, dates, parties):
[paste]

This invoice is: [one I am sending / one I received]. Currency: [ ]

Do this:
1. Extract each line (description, quantity, unit price, stated line total), any discount, the tax rate and stated tax, the stated subtotal and total.
2. Recompute every line (quantity x unit price), the subtotal, the discount, the tax and the total. If you can run code, use exact decimal arithmetic and say you did. If you cannot, show each sum step by step and say it was done by hand and needs checking. Never state a total from memory.
3. Compare each recomputed number with the stated one. List every difference with both numbers.
4. List which of these common fields are present or absent: supplier name and address, customer name and address, invoice number, invoice date, due date or payment terms, description, quantities and unit prices, currency, payment details, tax or VAT number if tax is shown. Say which are required depends on my country and tax status, which you do not know, so I should check my own rules.
5. Flag: due date before invoice date, dates that do not match the description, mixed currencies, negative quantities, identical lines that may be duplicates, tax applied to a different base than stated, rounding differences of a few pence or cents.

Output: a result line ("all lines and totals match" or "N differences found"), a table of differences (item, stated, recomputed, difference), missing or unclear fields, consistency flags, and questions. Do not correct the invoice silently. Write "not stated" for missing figures. Plain short sentences, no em dashes.

How to check the result

If your chatbot cannot run code, it does the sums by hand and says so. In that case, re-add the totals yourself with a calculator or a spreadsheet. This takes two minutes and it is the whole point of the check.

Look at the table of differences. Each row has the stated number and the recomputed one. A difference of a few pence or cents is often rounding, which the prompt flags separately. A bigger difference is a mistake in one of the two.

Read the missing fields list carefully. The prompt notes that which fields are required depends on your country and your tax status, which it does not know. Treat the list as a prompt to check your own rules, not as a ruling.

Then read the consistency flags: a due date earlier than the invoice date, mixed currencies, two identical lines that may be a duplicate. These are the problems people miss when they check only the total.

If you are paying the invoice, send the difference list back to the supplier. If you are sending it, fix the invoice and keep a note of what changed.

Limits

• It does no tax advice. It will not say what rate applies or whether tax is due.
• It only knows the invoice text you paste. It cannot check the work was done or the goods arrived.
• It never corrects the invoice silently. It lists differences for you to resolve.
• Scanned invoices need to be typed or copied out first.

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Want this as a ready-made skill? Invoice Checker (Free) does it in your own assistant.