How to Build an Invoice Approval Workflow That Stops Paying the Wrong Bills

Why invoice approval matters more than most teams think

Invoice fraud is more common than businesses expect. The most common types are not elaborate schemes but simple errors and opportunistic fraud: duplicate invoices from the same vendor, invoices for goods never delivered, invoices from vendors who changed their banking details (a classic email intercept scam), and, in small companies, payments authorized by people who should not have authority to approve them.

Beyond fraud, there is the problem of invoice-by-invoice spending decisions made by people with no budget context. When any manager can approve any invoice up to $10,000 without oversight, you end up with departments spending against budgets that finance cannot track until the month-end close.

An invoice approval workflow is the control layer that sits between receiving an invoice and paying it. Done right, it takes less time than the ad-hoc process it replaces and creates an audit trail that makes both internal reviews and external audits faster.

The basic structure of an invoice approval workflow

The workflow has four steps:

1. Invoice intake The invoice arrives (email, paper, EDI, vendor portal) and is captured into your AP system. Manual data entry is the highest-error step here, which is why most modern AP tools use OCR to extract invoice data automatically.

2. Three-way matching (or two-way for services) For goods: the invoice is matched against the purchase order (PO) and the receiving document. The quantities, prices, and vendor on all three documents should match. Discrepancies flag the invoice for manual review.

For services with no PO: the invoice is matched against the approved contract or SOW. This two-way match is less rigorous but better than no match at all.

3. Approval routing The invoice is routed to the right approver based on the rules you define. Common routing logic:

  • Under $500: approved by department manager
  • $500 to $5,000: approved by department manager and controller
  • Over $5,000: approved by VP or CFO
  • Any vendor not on the approved vendor list: requires finance review regardless of amount

4. Payment scheduling Approved invoices are scheduled for payment according to the vendor’s terms. Paying within terms (not early, not late) preserves your cash position and maintains vendor relationships.

Designing the right approval tiers for your company

There is no universal right answer for approval thresholds. The right tiers depend on your company size, spending volume, and risk tolerance.

For a company with $5M to $20M in annual revenue, a common structure:

  • Under $1,000: approved by the person who requested the purchase
  • $1,000 to $10,000: approved by department head
  • Over $10,000: approved by CFO or CEO

For a company with $1M to $5M in revenue:

  • Under $500: auto-approved if from an active vendor with a history of accurate invoices
  • $500 to $5,000: approved by owner or controller
  • Over $5,000: all-hands sign-off

The trap to avoid: requiring sign-off from too many people on small invoices. If your controller needs to approve a $47 invoice from your office supply vendor, you have created friction with no real risk benefit. Reserve human attention for the invoices where human judgment actually matters.

What to do when invoices arrive without a PO

In most growing companies, a significant share of invoices arrive without a purchase order because the company has not implemented a formal PO process. This creates a gap in the control structure because there is no pre-approved document to match the invoice against.

The practical fix is a retroactive approval process: the invoice goes to the manager of the department that received the goods or services, who confirms it was authorized and codes it to the right GL account. This is a manual step that should ideally be replaced by a PO process over time, but for most small and mid-size companies, retroactive approval is the practical near-term solution.

Tools that handle this well

For small businesses (under 200 invoices per month):

  • BILL handles approval routing with configurable rules. Approvers receive email notifications and can approve from their phone. Integrates with QuickBooks Online.
  • Melio is simpler and free for ACH but has very basic approval workflows (essentially just who can initiate payments, not multi-level approval logic).

For mid-market companies (200 to 2,000 invoices per month):

  • Stampli is built specifically around the approval workflow, with AI that routes based on past patterns and centralizes all invoice communication in one place.
  • Tipalti adds global payment capabilities alongside the approval workflow, useful for companies paying international vendors.

For enterprise:

  • Coupa, SAP Ariba, and Oracle Cloud Procurement handle complex multi-entity, multi-currency, and multi-level approval structures that mid-market tools cannot match.

Frequently asked questions

What is three-way matching in accounts payable? Three-way matching compares three documents before payment: the purchase order (what was ordered and at what price), the receiving document (what was actually delivered), and the vendor invoice (what the vendor is charging). Payment is only processed when all three documents agree. It is the most common control against overpayment and fraudulent invoices.

How long should invoice approval take? A well-designed workflow should get most invoices approved within 2 to 3 business days. Invoices that sit in approval queues for more than 5 business days create cash flow problems (early payment discounts get missed) and vendor relationship friction. If your approval cycle is regularly exceeding 5 days, look at where invoices are stalling in the queue.

What is the best way to prevent duplicate invoice payments? AP software with duplicate detection is the most reliable method. Most tools scan for invoices with the same vendor, amount, and date range and flag them before payment. Manual duplicate checks are error-prone. A basic rule in most AP software: never mark an invoice as paid manually in your accounting system until you can see the corresponding payment record.

Should approval workflows be different for recurring vendors versus new vendors? Yes. For active vendors with a history of accurate invoices, some companies use automatic approval for invoices below a threshold that match a previous invoice closely. For new vendors, or for vendors changing their banking details, additional scrutiny is warranted. Any request to change payment bank details should require direct phone verification, not just an email request, to guard against payment redirect fraud.