AnswersWhen a client won't pay
My invoice was rejected by a supplier portal (Coupa, Ariba). What went wrong?
The short answer
Usually a fixable mismatch. Portals commonly bounce an invoice because it does not match the buyer's order or records, such as a missing or closed purchase order number, a total above the order balance or the wrong billing entity. Read the reason, fix that one thing, resubmit the same day and note both dates. If no payment date was agreed, the 30-day clock runs from the later of delivery and the customer getting your invoice3, so a bounced invoice may not have started it.
What you can add
A £5,400 invoice, bounced then paid 25 days late
Say you invoiced another business £5,400 on 3 August 2026 on 30-day terms, so payment was due on 2 September. The portal rejected it on 6 August for a missing order number. You saw the message on 28 August and resubmitted that day. It was paid on 27 September, 25 days after the due date. For a debt that goes late between 1 July and 31 December 2026, the yearly rate is 11.75%8, which is £1.74 a day on this invoice. Treat the sum as a measure of the wait, not a claim.
Most of the delay sat in the 22 days between the rejection and the day it was seen. Check the portal for messages the day you submit. Whether those days can be claimed depends on your contract and on why the invoice sat.
The road ahead
Six steps. All free.
Step 1: Read the reason, then screenshot it
Open the rejection message before you change anything, and screenshot it with the date and time. If it is blank or only a code, ask the buyer's accounts payable team what it means.
Step 2: Match it to a common cause
A missing, closed or expired purchase order number is the common one. The buyer's requester has to extend or reissue the order. A total above the order balance, or lines that do not match the order, means matching the lines to the order, not cutting the sum.
Also common: the wrong billing entity or address, an invoice number already used, an invoice date outside the order period, VAT details that do not add up, a file type the portal does not take, and supplier set-up not finished, such as unverified bank details. Check the rest against what an invoice must include4.
Step 3: Fix that one thing and resubmit once
Change only what the message names, and resubmit the same day. Do not lower the total to fit the order: that changes what you are owed. If the portal blocks the old number, ask which number to use and say which invoice it replaces.
Step 4: Write down two dates
Note the day it bounced and the day it was accepted. If your contract names a payment date, that date rules, so read it first. If it names none, the 30 days start from the later of the day the work was done and the day the customer had notice of the amount1.
A rejected invoice may not have given that notice, so the start can only move later: at worst, to the day it was accepted. Work from that date, and treat the earlier start as the best case, not a promise.
Step 5: Tell accounts it is fixed
Email accounts payable and copy the person who ordered the work. Quote the message, say what you changed, give the invoice number and amount, and ask for confirmation it is accepted and the date it will be paid. Use the email below.
Step 6: Next time, ask before the work starts
Get the order number and the exact billing name and address in writing before you begin. The Small Business Commissioner5 says to ask for the order number early, because a late one delays the invoice and can delay the payment.
A week before the due date, ring accounts and ask whether the invoice has what it needs5. The Commissioner's own wording6 for the call: is there anything I can do to speed things through?
A letter you can copy
Email confirming a corrected invoice
Send it the day you resubmit an invoice that the portal rejected.
Tap a highlighted gap to see what goes in it.
Send it to the accounts payable address or the contact on the portal, and copy the person who ordered the work. Email is dated, so it stands as a record. Keep the rejection screenshot and a copy of the resubmitted invoice with it.
When not to bother
When it isn't worth it
- It was accepted and now says "approved". The portal is no longer the problem. The wait is the buyer's payment run, so ask accounts payable which run it is in and on what date.
- The real problem is a dispute over the work. A corrected invoice will not settle a disagreement about the work. Deal with that first, and keep the undisputed part moving.
- The contract does not make the portal the route. If it allows email, a clean emailed invoice may be quicker. If it names the portal, an emailed PDF may not count as sent, so read the invoice clause first.
- The bounce was your own mistake and you want interest for the delay. A court can cut or remove interest because of the supplier's conduct2, so treat that interest as a measure of the wait. A closed order on their side is not your conduct.
The Wolf's note
The portal compares your invoice with the buyer's order and records. When they disagree, a closed order number is theirs to reopen and a wrong billing name is yours to fix. Fix what is yours, ask for what is theirs, and write down both dates.
Mr. Wolf · the AI inside WolfX
What comes next
Your next move
Don't want to do this yourself?
The Wolf does every step for you. He works out the sums, writes the letters and keeps track of every date. Nothing goes without your yes.