Earlier this year, another stack of indexed invoices landed on your desk. Cleaning, security, catering, grounds maintenance: each with an amount slightly higher than last year. The question was probably not whether you needed to check them, but whether you had the time. That is exactly where money leaks away. Invoice verification against the contract is the only way to know whether the amount you pay is also the amount that is allowed.
Why your invoices are rising visibly this year
According to Statistics Netherlands (CBS), collective labour agreement wages rose by 4.0 percent year on year in July 2026. Many contracts for outsourced services index on the basis of that wage development. This year, suppliers are therefore passing on a markedly higher index than you were used to.
The problem is not the fact that indexation happens. That is usually agreed in the contract. The problem is that an error in that indexation compounds over time. If a supplier passes on 4.0 percent where the contract prescribes a lower percentage or a different reference date, that error raises your base amount. The following year, indexation is applied again on top of that inflated base amount. An error of 1 percent in 2026 is still there in 2029.
What goes wrong with manual checks?
In practice, most organisations rely on the supplier. “They will have calculated it correctly.” That is understandable, because looking up the indexation clause and recalculating every invoice takes time you do not have. But that trust is precisely where money leaks away.
The errors that go unnoticed during manual checks are often subtle:
- The wrong index applied: collective labour agreement wages where the contract prescribes CPI, or the other way around
- The index calculated on the wrong base amount, including earlier additional work that should not be indexed
- Indexation applied before the contractual effective date
- A double charge, where an increase appears in two consecutive invoices
Excel and the PDF folder fall short here. They contain no logic that automatically checks an invoice line against the agreement. And they leave no trace of why you approved an invoice or not. Anyone asked two years later how an amount came about is left empty-handed.
What exactly do you check on a supplier invoice?
Verifying an invoice against the contract means comparing it line by line with what was agreed. Take a cleaning invoice for an organisation with 40 sites. On each line you check:
- Hourly rates: does the rate used match the contract?
- Price per m²: is the price per square metre times the agreed area correct?
- Quantities: are the agreed volumes and frequencies invoiced, and no more?
- Discounts and minimum purchases: are negotiated agreements applied?
- Indexation percentage: does the percentage charged equal the contractual index?
- Effective date: does indexation take effect on the date stated in the contract?
- Additional work: was extra work agreed in advance, and is it booked correctly and separately?
Suppose the supplier charges 4.0 percent. The contract prescribes an index of 3.2 percent, with an effective date of 1 April rather than 1 January. A small difference per line. Across 40 sites and a full year, it adds up, and it carries over into next year’s base amount.
Accounting-correct is not the same as contract-correct
This is the heart of the problem. Finance sees an amount, checks whether it adds up and whether there is budget for it, and books it away. The invoice is then correct from an accounting point of view. But whether the amount is allowed under the contract is something finance does not see. That is a different question, with a different source.
Contract management checks not whether the amount adds up, but whether the amount is permitted. For that you need the contract as your source: the rates, the indexation clause, the agreed quantities. Automated checking is only possible if those agreements are recorded in a structured way. Not in a PDF on a network drive, and not in the head of the colleague who once negotiated the contract, but as data your system can read.
How does invoice verification work when the contract is the basis?
Automated invoice verification only works when the contractual agreements are already in your system. That is the difference between GRIP and a standalone invoice scanner. A scanner reads what the invoice says. GRIP checks that invoice against the agreements it already knows: the indexation clause, the rates, the quantities and the SLA terms.
Because the contract is the source, the system flags when an invoice line deviates. An indexation that is 0.8 percent too high, an effective date that is wrong, a rate that does not match: it stands out without you having to look up the clause yourself. This saves time and catches the errors you would miss with manual checks. In Financial management, you see the complex calculations, indexations and invoicing in one place.
Getting a grip on what you pay: audit trail and roles
Invoice verification is not only about avoiding overpayment. You must also be able to show why you approved an invoice. During an audit or a query from the CFO, you want to demonstrate that every euro was paid in line with the contract.
For that, it must be clear who does what. The contract manager flags the deviation and checks it against the agreement. Finance verifies the amount and the budget line. The budget holder gives final approval. Once those steps are recorded, a trail emerges: which invoice was checked, against which clause, and why it was approved or not. The fact that GRIP is ISO 27001 certified means that this recording happens securely and in line with fixed information security standards.
Frequently asked questions
How much can a supplier index in 2026?
That depends on the indexation clause in your contract, not on a fixed percentage. Many contracts are linked to CBS collective labour agreement wages, which rose by 4.0 percent in July 2026. Other contracts use the CPI or their own formula. Always check which index, reference date and effective date your contract prescribes.
What exactly is invoice verification against the contract?
It is the line-by-line comparison of a supplier invoice with the contractual agreements: rates, prices per m², quantities, discounts, indexation percentage and effective date. You check not whether the amount adds up, but whether the amount is allowed under the contract.
Why does Excel not work for invoice verification?
Excel contains no logic that automatically checks an invoice line against the contractual agreement, scales poorly across many sites and suppliers, and leaves no audit trail. Errors such as an incorrectly applied index or a double charge therefore go unnoticed.
What is the difference between accounting-correct and contract-correct?
Accounting-correct means the amount adds up and there is budget for it. Contract-correct means the amount is permitted under the agreements. Finance sees the amount, contract management sees whether the amount is allowed.
Why does an indexation error carry on for years?
An indexation that is too high raises your base amount. The following year, indexation is applied again on top of that inflated base amount. An error of 1 percent in 2026 is therefore still there in 2029, and adds up cumulatively.
Can you automate invoice verification?
Yes, provided the contractual agreements are recorded in your system in a structured way. The system then checks every invoice line against the known rates, indexation clause and quantities, and flags deviations without you having to look up the clause yourself.
Want to get a grip on what you pay? See how GRIP solves this in Financial management.