If you see this error, it means the invoice number already exists somewhere in Xero. It could match a Fergus customer invoice, a Fergus supplier invoice, or something typed directly into Xero — like a supplier bill.
Log into Xero and search for that invoice number. Check active invoices, deleted or voided invoices, and supplier bills — it's not always another customer invoice causing the clash.
Step 1: Was that number entered directly in Xero, not sent from Fergus?
For example, a supplier bill someone typed straight into Xero rather than one that came from Fergus.
If so, this is the quick fix: edit that number in Xero, adding a few extra letters to the end so it's different (e.g. change INV-1234 to INV-1234-B). This frees up the number so your Fergus invoice can go through. You don't need to touch anything in Fergus.
Step 2: If the matching number is an old Fergus invoice
If it's a supplier invoice
Go to Supplier Docs, then open the Supplier Document Report.
Use the search box to find the invoice by its number.
Click the three dots (⋮) next to it and choose Edit Invoice.
Change the invoice number slightly so it's different from the one in Xero, then click Save Changes.
Then send it again
Fixing the number doesn't resend it automatically — you'll need to do that yourself:
For a customer invoice: go to the job's Invoicing tab, click the three dots (⋮), and choose Resend to Xero.
For a supplier invoice: go to the job's Supplier Documents tab, click the three dots (⋮), and choose Send To Xero.
If it's a customer invoice
Once a customer invoice is approved in Fergus, it's locked — even the invoice number can't be changed. That's because it's already been sent (or tried to send) to your accounting software.
The fix is to create a new one:
Open the invoice, click the three dots (⋮), and choose Duplicate Invoice.
This creates a new invoice with a new number.
Approve it to send to Xero.
If this keeps happening, your starting invoice number in Fergus might be too close to numbers already used in Xero. Get in touch with our Support team with the highest invoice number currently in your Xero account, and we'll update your starting number in Fergus so it stops coming up.