Construction and project-services firms bill by milestone, not by delivery date: 20% on mobilisation, 30% at foundation, the balance across defined stages, with 5-10% retention held on every certificate until defects liability ends. ERPNext ships a Contract DocType and payment schedules on the Sales Invoice, but the two never talk to each other. There is no native ledger that says "this contract is worth 4.2M, we have certified 2.6M across five milestones, invoiced 2.4M, collected 2.1M, and are holding 210K in retention due for release in March." Finance teams end up rebuilding that picture in spreadsheets, and retention amounts silently fall through the cracks because nothing in ERPNext tracks a held amount as a distinct, releasable obligation.

We build a proper Frappe app that closes this gap. A new Contract Billing Plan DocType links to the standard Contract and holds a child table of milestones (description, planned value or percentage, planned date, status, retention percentage). A whitelisted server method generates a draft Sales Invoice from a milestone in one click, stamping the contract and milestone references onto the invoice, applying the retention deduction as a line against a configurable retention-receivable account, and writing the certified value back to the plan. hooks.py doc_events on Sales Invoice and Payment Entry (on_submit, on_cancel) keep certified, invoiced, paid, and retention-held figures in sync on the contract, so cancelling an invoice cleanly reverses the milestone status instead of leaving stale totals. A separate retention-release workflow (a Retention Release DocType with a Frappe Workflow: Draft to Reviewed to Approved) generates the final retention invoice or credit only when the release condition is approved, with role-gated approval.

On top of the data model we build the reports and controls finance actually asks for: an "Outstanding by Contract" query report (contract value, certified, invoiced, paid, retention held, balance to bill, overdue), a client script on the Sales Invoice that surfaces the linked contract and remaining billable balance inline, permission rules and a "Contract Billing" role profile so project managers can raise milestone invoices but only finance approves retention release, and a scheduler event that flags milestones whose planned date has passed without certification. Everything is exposed over the Frappe REST API and whitelisted methods so your project system or a client portal can read contract billing status programmatically. The app targets ERPNext v15 and is tested for the v16 line.

Because this is build-to-order, nothing is a black-box download. After a short scoping call we confirm your milestone and retention rules, chart-of-accounts mapping, and approval roles in a written scope, then build against your requirements. You get the installable app source for your version, install and configuration on your bench, UAT on a staging site with a rollback plan, and a post-go-live support window. Typical delivery is one working week from confirmed scope.