The billing system a finance person is building

175 statements of work reconciled

One month's run, scored line by line against the invoices that actually went out. This is the test it has to keep passing, month after month, before it earns the right to replace anything.

What it does

Paid media

Imports a monthly ad spend report covering roughly thirty client campaigns, matches each spend line to a client, and computes the management fee from that client's actual contract terms — flat percentages, tiered brackets, minimums, maximums, annual caps, and parent/child accounts that share an aggregate fee. Umbrella accounts needing their spend split across child campaigns are caught at intake and can't reach invoicing until someone resolves the split.

Projects & retainers

Ingests worklogs, maps every hour to a department and a statement of work, and gives each project manager one review screen: what was logged, what was billed, and the decision the month turns on. Retainers carry hour banks that roll month to month with over- and under-burn tracked and every rolling balance auditable back through its history. Projects get a three-outcome decision — value bill, billable overage, or write-off — with the maths already done for the PM.

The finance layer

Department-level revenue recognition moves are captured on each monthly review and roll up into a balanced journal entry. Every correction, override, sign-off and decision is logged with who, when, and why. That's the subledger the spreadsheet never was.

Will it work

        Parallel run · June
        108 exact tiesStatements of work reconciled against the actual QuickBooks invoices, with four variances — every one explained and agreed.
      
        Paid media · July
        Matched to the pennyComputed fees tied to the manual workbook across 31 clients — and the import validation caught a dropped spend row the manual process had missed.
      
        Revenue recognition
        A balanced draftPreviously assembled by hand from notes scattered through the billing sheet. Now generated with every line traceable to its source row.

Encouraging, not conclusive. Two months of scored runs isn't proof, and the variances that turned up are the point of running it this way — each one is either a bug to fix or something the old process was getting wrong.

The result I'd point at is the dropped spend row. That month the app wasn't just reproducing the spreadsheet — it found something the spreadsheet had quietly got wrong.

How it's being built

Stack Next.js 14 (App Router) · TypeScript · Prisma · SQLite
Integrations QuickBooks Online for invoices and the chart of accounts; Jira for worklogs
Fee engine Decimal-safe money math with a backtest suite that replays historical months and asserts the computed fees tie to what was actually invoiced
Data Real client data never entered the repository — fixtures are synthetic and live data stays in gitignored local stores

Every month-end close is a test suite. When the app and the spreadsheet disagree, we chase the variance until one of them turns out to be wrong. Several times now it's been the spreadsheet — which is the whole argument for building this.

Where it goes from here

The work continues each month. The direction we're heading is to fold this functionality into the agency's existing operations platform, so there's one system rather than two — with this app serving as the working spec and the reference implementation for that build.

I'll take that trade. What I'm after is a billing cycle with a real audit trail behind it instead of a spreadsheet's version history — and that survives the move.

Built with Claude Code. Reviewed, reconciled, and argued with by a finance person who now can't stop shipping software. One of several things built for the agency.