Change order: what it actually authorizes on an invoice

Glossary page defining "change order" in AP/contract audit context, covering how it creates margin drift, verification steps, and links to related drift-type.

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Change order: what it actually authorizes on an invoice

Margin drift is the gap between what a vendor contract says and what the invoice actually charges. A change order is one of the few mechanisms that legitimately moves that baseline: it is a written amendment to a contract or purchase order, and once signed it becomes part of what the invoice should be checked against.\n\nThe risk is not the change order itself. It is invoices that reference one that was never signed, has already expired, or covers different scope than what was billed.

1. What makes a change order valid?

A valid change order is a written amendment, signed by whoever the base contract names as the authorized approver, that specifies the exact scope, price, and duration it modifies. Anything short of that, a verbal approval, an email from someone outside the approval chain, or a document missing a price, does not amend the contract. The invoice should be checked against the base terms until a valid change order says otherwise.

Verification means locating the signed document and confirming the approver matches the contract's named authority before treating any invoice line as covered by it.

2. How does a change order create margin drift?

Drift appears when billing outruns the document. An invoice can cite a change order number that was never signed, apply a modified rate past its stated end date, or bill scope broader than what the change order describes. In each case the invoice is charging against an amendment that, in whole or in part, does not authorize what was billed.

The base contract still governs everything the change order did not specifically modify.

A change order that adds one task to a maintenance contract does not touch the rate card the rest of the invoice should still follow.

3. How do you verify a change order against an invoice?

Pull the signed change order, confirm its effective and end dates against the invoice date, and match every line billed under it to the scope it names. Anything on the invoice outside that named scope reverts to the base contract or purchase order terms. A change order that raises a rate for one task does not authorize a higher rate on tasks it never mentions.

Confirm the invoice date falls between the change order's effective date and any stated expiration or completion trigger, not just after signature. Then line up each billed task against the described scope: a task not named there bills under the original contract, not the amendment.

4. Who should own change order tracking?

The approver named in the master contract, usually a procurement lead or the cost center owner, should hold the authoritative log of every signed change order, its dates, and its scope. AP should never accept a change order reference on an invoice without checking it against that log. A vendor citing a change order number AP cannot locate is a hold, not an approval.

Without a shared log, verification depends on trusting the vendor's own citation, which is exactly the gap a diagnostic is built to close.

For the wider pattern this sits inside, start with the margin drift guide. See also margin drift vs. legitimate price increases: how to tell them apart and accessorial charge audit: the surcharges nobody validates.

5. Frequently Asked Questions (People Also Ask)

What is a change order?

A change order is a written modification to an existing contract or purchase order that adds, removes, or alters scope, price, or timeline. It supersedes the original terms only for the items it specifically covers. Everything else in the base contract or PO still applies and still governs what the vendor may invoice.

Does a change order need a signature to be valid?

Most master service agreements require written authorization from a named approver before a change order takes effect. A verbal instruction from a site manager to a vendor technician is not a change order under those terms, even if work proceeds. Invoices billed against unsigned or unauthorized change orders are billed outside contract.

Can a vendor invoice before a change order is signed?

A vendor can submit an invoice any time, but that does not make the invoice compliant. If the contract requires signed authorization before billable work begins, an invoice for unsigned change order work is billed against a document that does not yet exist and should be held for verification.

How is a change order different from a purchase order?

A purchase order authorizes a defined purchase under existing terms. A change order modifies those terms or that scope after the fact: a new rate, an added task, an extended timeline. The two work together. The change order amends what the purchase order or master agreement originally authorized.

What happens if a change order expires but billing continues?

Many change orders carry an end date or a completion trigger tied to the modified scope. If billing continues past that date at the change order rate rather than reverting to the base contract rate, the invoice is charging for authorization that no longer exists.

Who should approve a change order internally?

The approval chain should match the authority named in the master contract, typically a procurement lead or the budget owner for the affected cost center. An invoice that references a change order approved by someone without that authority is a control gap even if the work was legitimately performed.

Does every scope addition require a formal change order?

Contract terms usually specify a dollar threshold or scope type below which informal approval is acceptable. Above that threshold, a formal change order is the only instrument that modifies the contract. Invoices for above-threshold work billed without one are billing scope the contract never authorized.

1. What makes a change order valid?

A valid change order is a written amendment, signed by whoever the base contract names as the authorized approver, that specifies the exact scope, price, and duration it modifies. Anything short of that, a verbal approval, an email from someone outside the approval chain, or a document missing a price, does not amend the contract. The invoice should be checked against the base terms until a valid change order says otherwise. Verification means locating the signed document and confirming the approver matches the contract's named authority before treating any invoice line as covered by it.

2. How does a change order create margin drift?

Drift appears when billing outruns the document. An invoice can cite a change order number that was never signed, apply a modified rate past its stated end date, or bill scope broader than what the change order describes. In each case the invoice is charging against an amendment that, in whole or in part, does not authorize what was billed. The base contract still governs everything the change order did not specifically modify. A change order that adds one task to a maintenance contract does not touch the [rate card](/glossary/rate-card) the rest of the invoice should still follow.

3. How do you verify a change order against an invoice?

Pull the signed change order, confirm its effective and end dates against the invoice date, and match every line billed under it to the scope it names. Anything on the invoice outside that named scope reverts to the base contract or purchase order terms. A change order that raises a rate for one task does not authorize a higher rate on tasks it never mentions. Confirm the invoice date falls between the change order's effective date and any stated expiration or completion trigger, not just after signature. Then line up each billed task against the described scope: a task not named there bills under the original contract, not the amendment.

4. Who should own change order tracking?

The approver named in the master contract, usually a procurement lead or the cost center owner, should hold the authoritative log of every signed change order, its dates, and its scope. AP should never accept a change order reference on an invoice without checking it against that log. A vendor citing a change order number AP cannot locate is a hold, not an approval. Without a shared log, verification depends on trusting the vendor's own citation, which is exactly the gap a diagnostic is built to close. For the wider pattern this sits inside, start with the [margin drift](/insights/margin-drift-spend-leakage-guide) guide. See also [margin drift vs. legitimate price increases: how to tell them apart](/guides/margin-drift-vs-legitimate-price-increases-how-to-tell-them) and [accessorial charge audit: the surcharges nobody validates](/guides/accessorial-charge-audit-the-surcharges-nobody-validates).

Questions & Answers

What is a change order?

A change order is a written modification to an existing contract or purchase order that adds, removes, or alters scope, price, or timeline. It supersedes the original terms only for the items it specifically covers. Everything else in the base contract or PO still applies and still governs what the vendor may invoice.

Does a change order need a signature to be valid?

Most master service agreements require written authorization from a named approver before a change order takes effect. A verbal instruction from a site manager to a vendor technician is not a change order under those terms, even if work proceeds. Invoices billed against unsigned or unauthorized change orders are billed outside contract.

Can a vendor invoice before a change order is signed?

A vendor can submit an invoice any time, but that does not make the invoice compliant. If the contract requires signed authorization before billable work begins, an invoice for unsigned change order work is billed against a document that does not yet exist and should be held for verification.

How is a change order different from a purchase order?

A purchase order authorizes a defined purchase under existing terms. A change order modifies those terms or that scope after the fact: a new rate, an added task, an extended timeline. The two work together. The change order amends what the purchase order or master agreement originally authorized.

What happens if a change order expires but billing continues?

Many change orders carry an end date or a completion trigger tied to the modified scope. If billing continues past that date at the change order rate rather than reverting to the base contract rate, the invoice is charging for authorization that no longer exists.

Margin Drift Resources