How Do You Automate Expense and Purchase Approval Workflows?

Detailed Explanation

Most small and mid-sized businesses run approvals the same informal way for years: someone emails a manager, or messages them on Slack or Teams, and waits — sometimes minutes, sometimes days, sometimes the request just sits until someone chases it. Approval workflow automation replaces that ad hoc chasing with a defined, rule-based routing system: a request is submitted once, the system decides who needs to approve it based on rules you set, and it tracks the request until someone acts.

A typical automated approval workflow has four parts:

1. Submission — a request (a purchase, an expense, a budget line) is entered through a form, an expense app, or a procurement tool, capturing the amount, category, and requester.
2. Routing rules — the system checks the request against defined conditions: amount thresholds, spending category, department, or requester role, and decides who needs to approve it, if anyone.
3. Approval or auto-approval — requests that fall under a low-risk threshold and meet defined conditions can clear automatically with no human step; everything else routes to the named approver, who gets a notification with enough context to decide without hunting for details.
4. Escalation and tracking — if an approver doesn't act within a set time, the system sends a reminder, and if it still sits unactioned, escalates to a second approver or a manager, so a request never silently stalls because one person is on leave or simply missed the notification.

This is a different layer from processing the resulting spend. Invoice processing automation captures and matches an invoice that's already arrived from a supplier; approval workflow automation governs whether the spend should happen at all, and who has to sign off before it does. The two commonly connect — an approved purchase order becomes the reference an invoice later gets three-way matched against. The equivalent approval-and-chasing logic also runs on the revenue side of the business — see how do you automate invoicing customers and following up on late payments for accounts receivable.

The same routing-and-escalation pattern applies to a genuinely different subject: reviewing and approving employee hours rather than spend — see how do you automate employee time tracking and timesheet approval.

Designing Your Approval Thresholds and Routing Rules

1. Start with your actual spending pattern, not a guess. Pull three to six months of expense or purchase data and look at the real distribution of amounts and categories — most businesses find the bulk of requests cluster at low values, with a small number of larger, genuinely risk-worthy ones. Set your auto-approve threshold from this data, not from a round number that feels reasonable.

2. Define routing by more than just amount. Amount is the most common rule, but category and vendor status matter too: a request to a new, never-used vendor is a different risk than a repeat order from an approved supplier, even at the same dollar value — route new-vendor requests to a person regardless of amount, at least for the first few orders.

3. Set escalation timeouts deliberately. A request with no response after 24 or 48 business hours (adjust to your business's pace) should automatically remind the approver, and after a further defined period, escalate to a backup approver. Without this, a single unavailable approver becomes a bottleneck for everyone waiting behind them.

4. Keep segregation of duties explicit in the rules. The person requesting a purchase should never be able to approve their own request, and for larger amounts, many businesses require two approvers rather than one — encode this as a hard rule in the workflow, not a policy people are trusted to remember.

5. Build it on what you already have before buying new software. If you're a Microsoft 365 business, Power Automate can build a full approval flow — form submission, routing by amount, approver notification via Outlook or Teams, and escalation — using its built-in Approvals connector, often without a separate procurement tool. Many accounting and expense platforms also include native approval routing; check what you already pay for before adding another system. The same Approvals connector pattern also applies to documents rather than spend requests — see how do you automate document workflows in OneDrive and SharePoint if what you're routing for sign-off is a file rather than a purchase.

Things to Consider

• A threshold set too low creates the exact bottleneck you're trying to remove. If almost every request needs a human approver, you haven't automated much — the point is to remove the low-risk majority from anyone's inbox, not to digitise every request equally.
• A threshold set too high is a control gap, not a convenience. Setting auto-approval high enough that meaningful spend clears with no review at all defeats the purpose of having approvals in the first place — calibrate against your actual risk tolerance, not just what reduces approver workload the most.
• Notifications need to carry enough context to act on. An approval request that just says "approve or reject?" with no amount, vendor, or reason forces the approver to go digging elsewhere, which is exactly the friction that gets requests ignored. Include what they need to decide in the notification itself.
• Where the approver sees the request matters as much as what's in it. A Microsoft 365 business can deliver the approval directly into Teams rather than email — see how do you automate approvals and notifications in Microsoft Teams for how that changes response speed.
• New vendors and one-off purchases deserve a human look regardless of amount, at least until there's a track record — a small but recurring risk (a fraudulent invoice from a never-verified supplier, a one-off contract with unfavourable terms) is easy to miss if the rule only looks at dollar value. A vendor that's been properly onboarded and verified up front — see how do you automate vendor and supplier onboarding for a small business — carries less of this risk than one added on the fly to push through a one-off purchase.
• Review your thresholds periodically, not once. What counted as a large purchase when the workflow was built can drift as the business grows — revisit the numbers roughly annually or after any noticeable change in spending volume.
• An approved purchase still has to land correctly in the books afterward. Once a purchase clears approval, it becomes just another transaction that needs categorizing — see how do you automate bookkeeping with bank feeds and rules for the automation that picks it up from there.
• This workflow is for purchases and vendor spend, not staff pay. Employee compensation runs through a separate, compliance-driven process — see how do you automate payroll for a small business for that distinct pipeline.
• Mileage and travel claims often route through this same approval layer, even though they're generated differently. See how do you automate mileage and travel expense tracking for how those claims are calculated before they reach the approval step covered here.

Common Mistakes

• Copying a generic threshold from another business instead of using your own data. A $500 auto-approve limit might be sensible for one company and reckless for another depending on typical spend and risk tolerance — base it on your actual numbers.
• Letting requesters approve their own requests. Even informally, this undermines the entire point of an approval control — enforce segregation of duties as a rule the system checks, not a norm people are expected to follow.
• No escalation path when an approver is unavailable. A workflow that just waits indefinitely for one named person creates exactly the kind of stalled request the automation was meant to prevent.
• Automating approval routing without automating the notification content. A technically-working workflow that pings someone with no usable detail just moves the friction from "who do I ask" to "what am I even approving" — it still gets ignored or rubber-stamped without real review.
• Treating the workflow as a one-time build. Spending patterns, vendor relationships, and business size change — a workflow tuned once at launch and never revisited tends to drift out of step with what actually needs review a year or two later.