The models move risk in different ways
The billing model is part of project design. It determines which records matter, when a draft invoice can be created, and where overruns appear. Selecting it after delivery begins usually leaves operations reconstructing the commercial agreement from email.
| Model | Primary evidence | Invoice trigger | Main delivery risk |
|---|---|---|---|
| Fixed fee | Scope, budget, change record | Date, schedule, or completion | Supplier absorbs excess effort |
| Time and materials | Approved billable work logs | Approved period or billing cycle | Customer carries effort variability |
| Milestone | Accepted deliverable or outcome | Milestone approval | Acceptance and dependency delays |
| Retainer | Agreement plus usage or availability record | Recurring schedule | Unclear rollover and scope boundaries |
| Non-billable | Time and cost record | No customer invoice | Internal cost becomes invisible if untracked |
Fixed fee requires a living scope boundary
A fixed price does not remove the need to record time. Hours show whether the delivery plan is consuming the budget faster than expected and whether future estimates should change. The project should preserve the agreed scope, internal effort budget, expected margin, change requests, and invoice schedule.
When the client requests additional work, decide whether it replaces existing scope, becomes a priced change, or moves to another project. Quietly absorbing it teaches both the client and the delivery team that the written boundary is optional.
Time and materials depends on defensible entries
The commercial record is the approved work itself. Each entry needs the correct person, rate basis, project, task, duration, billable status, and enough context to survive client review. Rate changes should be effective-dated rather than overwriting history.
Set billing cutoffs. Late time can move to the next invoice or reopen a period through a controlled process. Editing an issued invoice to force new hours into it weakens both the audit trail and the customer explanation.
Retainers need explicit consumption rules
A retainer can purchase recurring deliverables, a bank of hours, priority access, reserved capacity, or a combination. State what is included, whether unused value rolls over, how overages are priced, and which work consumes the balance.
Track the opening balance, approved consumption, adjustments, expiry, and remaining amount in the same unit used by the agreement. If the contract is monetary, do not present an hour balance without the rate logic that produced it.
Mix models deliberately, not inside one ambiguous total
A discovery phase may be time and materials, implementation fixed fee, and support a monthly retainer. That combination is workable when each phase has its own budget, evidence, and invoice rule. Mixing them inside one undifferentiated project makes margin and remaining obligation difficult to explain.
Billing operations are not the same as accounting treatment. IFRS 15, for example, bases revenue recognition on contracts and performance obligations. Firms should have qualified accountants determine recognition, tax, and reporting policy; the project system should preserve the operational evidence they need.
Sources and scope
These guides explain operating practice, not legal, tax, employment, or accounting advice. The primary sources below support the regulatory or professional context referenced in this article.
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