Invoice PO & GRN Matching n8n Workflow: Gmail, OpenAI, Slack

Automate Invoice-to-PO/GRN 3-Way Matching with Gmail, OpenAI & Slack

Stop manual invoice checking. This n8n workflow automatically reads vendor invoices from Gmail, uses OpenAI to extract structured fields, matches each invoice against Purchase Orders (PO) and GRNs in Google Sheets, and routes approvals via Slack—while archiving documents to Google Drive and generating a weekly bank-upload CSV batch.

What this workflow does

• Hourly intake from Gmail: Triggers every hour when Gmail receives invoice attachments under the invoices label that are not yet marked processed.
• Document handling + OCR-free extraction: Splits PDF/image attachments, uploads each file to Google Drive, and sends the document to OpenAI to extract vendor details (including GSTINs), PO number, line items, and taxes.
• 3-way matching in Google Sheets: Loads vendor master data, POs, GRNs, and the existing AP ledger from Google Sheets, then performs invoice matching with GST/TDS validation and duplicate detection.
• Writes results back to the AP ledger: Updates the invoice record with status, exceptions/warnings, an approval token, and the Drive link.
• Slack routing: Posts FYI for auto-approved invoices; sends approve/reject actions for invoices requiring review; drafts a Gmail vendor query and posts a hold notification to Slack for invoices that fail checks.
• Approval via webhook: Accepts approve/reject clicks, validates the one-time token against the AP ledger, updates status, and posts the decision to Slack.
• Weekly payment batch: Every Friday at 11:00 IST, it reads approved invoices due in the next seven days and prepares a payment CSV batch for bank upload.

Use cases

• AP teams reducing cycle time for invoice processing using PO & GRN matching.
• Automation engineers implementing a review/hold workflow with Slack approval links.
• SaaS operators needing scalable invoice intake, archiving, and weekly payment exports.

Technical details

• Nodes/controls: if, code, gmail, slack, switch, webhook
• Core systems: Gmail (trigger + hold/query emails), OpenAI (invoice field extraction), Google Sheets (AP ledger, PO/GRN data), Google Drive (archival), Slack (FYI + approvals)