n8n Invoice Duplicate & PO Mismatch Checker (Gmail, Sheets, OpenAI)

Automatically detect duplicate invoices and PO mismatches from Gmail PDFs—then route exceptions for approval

This n8n workflow monitors your Gmail for PDF invoices, extracts invoice fields with OpenAI, and cross-checks them against your Google Sheets registers to catch duplicates, bank-detail changes, and PO/receiving mismatches. High-risk invoices get put on hold; clean invoices are auto-cleared; unreadable PDFs are flagged for manual keying.

What this workflow does

• Runs every 5 minutes when a new Gmail message arrives that includes a PDF attachment, loading configuration values such as Google Sheet ID, approver emails, and thresholds.
• Reuses Google Sheets registers by reading the Suppliers, Purchase orders, and Ledger tabs once for the whole batch of emails.
• Processes each invoice independently by splitting emails into one item per PDF attachment.
• Extracts structured invoice data with OpenAI (gpt-4o-mini), including supplier, invoice number, dates, totals, PO number, and IBAN.
• Validates invoice consistency by parsing dates, checking totals arithmetic, implying VAT rate, and performing an IBAN checksum.
• Checks against registers for prior payment/duplicates, supplier bank-detail changes, and PO/received-amount mismatches.
• Routes the result via email: holds blocked invoices for finance, sends a Gmail approval request and waits up to 3 days for higher-risk/higher-value invoices, auto-clears small clean invoices, and flags unreadable invoices for manual keying—then updates/appends the Ledge in Google Sheets.

Use cases

• Catch duplicate invoice submissions before they hit payment.
• Detect changes in IBAN/bank details versus your supplier master.
• Confirm PO reference and received amounts match expected purchase orders.
• Keep finance reviews focused by routing exceptions and clearing low-risk invoices automatically.

Technical details

• Integrations/nodes: Gmail, Google Sheets, OpenAI (gpt-4o-mini), plus workflow logic nodes such as if, set, code, switch, and no op.
• Core flow: Gmail trigger → PDF text extraction → OpenAI structured extraction → validation (totals/VAT/IBAN) → cross-check registers in Sheets → email routing (hold/approval/auto-clear) → update Sheets Ledger.