diff --git a/apps/hq/src/api.ts b/apps/hq/src/api.ts
index 9c20d8e..9eee431 100644
--- a/apps/hq/src/api.ts
+++ b/apps/hq/src/api.ts
@@ -731,7 +731,7 @@ export function apiRouter(
       res.status(404).json({ ok: false, error: 'Client module not found' }); return
     }
     try {
-      const body = req.body as { key?: unknown; done?: unknown; doneOn?: unknown; label?: unknown }
+      const body = req.body as { key?: unknown; done?: unknown; doneOn?: unknown; label?: unknown; paymentId?: unknown }
       if (typeof body.label === 'string') {
         // Add a project-specific milestone.
         res.json({ ok: true, milestones: await addProjectMilestone(db, staffId(res), id, body.label) })
@@ -741,7 +741,8 @@ export function apiRouter(
         throw new Error('Provide { key, done } to tick a milestone, or { label } to add one')
       }
       const milestones = await setMilestone(db, staffId(res), id, body.key, body.done,
-        typeof body.doneOn === 'string' ? body.doneOn : undefined)
+        typeof body.doneOn === 'string' ? body.doneOn : undefined,
+        typeof body.paymentId === 'string' ? body.paymentId : undefined)
       res.json({ ok: true, milestones })
     } catch (err) {
       res.status(400).json({ ok: false, error: err instanceof Error ? err.message : String(err) })
diff --git a/apps/hq/src/db.ts b/apps/hq/src/db.ts
index cd0cef8..8ba8410 100644
--- a/apps/hq/src/db.ts
+++ b/apps/hq/src/db.ts
@@ -516,6 +516,11 @@ function migrate(db: SqliteRaw): void {
   if (!modCols2.some((c) => c.name === 'sort_order')) {
     db.exec(`ALTER TABLE module ADD COLUMN sort_order INTEGER NOT NULL DEFAULT 100`)
   }
+  // Client Detail redesign task 4: link an advance/balance payment-step tick to the real payment.
+  const milestoneCols = db.prepare(`PRAGMA table_info(project_milestone)`).all() as { name: string }[]
+  if (!milestoneCols.some((c) => c.name === 'payment_id')) {
+    db.exec(`ALTER TABLE project_milestone ADD COLUMN payment_id TEXT`)
+  }
   rebuildStaffUserRoleCheck(db)
   rebuildReminderRuleKindCheck(db)
 }
diff --git a/apps/hq/src/migrations-pg.ts b/apps/hq/src/migrations-pg.ts
index 36756e3..54abbd0 100644
--- a/apps/hq/src/migrations-pg.ts
+++ b/apps/hq/src/migrations-pg.ts
@@ -352,4 +352,9 @@ UPDATE client SET category = CASE
 END;
 `,
   },
+  {
+    // Client Detail redesign task 4: link an advance/balance payment-step tick to the real payment.
+    id: '015-milestone-payment-id',
+    sql: `ALTER TABLE project_milestone ADD COLUMN IF NOT EXISTS payment_id text;`,
+  },
 ]
diff --git a/apps/hq/src/repos-milestones.ts b/apps/hq/src/repos-milestones.ts
index 0703a51..93e5e52 100644
--- a/apps/hq/src/repos-milestones.ts
+++ b/apps/hq/src/repos-milestones.ts
@@ -9,8 +9,12 @@ import type { DB } from './db'
  * across the whole book: "who's pending go-live", "who hasn't paid the advance".
  */
 
-export interface Milestone { key: string; label: string; done: boolean; doneOn: string | null; sort: number }
-interface MilestoneRow { key: string; label: string; done: number; done_on: string | null; sort: number }
+export interface Milestone {
+  key: string; label: string; done: boolean; doneOn: string | null; sort: number; paymentId: string | null
+}
+interface MilestoneRow {
+  key: string; label: string; done: number; done_on: string | null; sort: number; payment_id: string | null
+}
 
 export interface TemplateEntry { key: string; label: string }
 
@@ -92,32 +96,52 @@ export async function ensureProjectMilestones(db: DB, clientModuleId: string): P
 
 export async function listProjectMilestones(db: DB, clientModuleId: string): Promise<Milestone[]> {
   const rows = await db.all<MilestoneRow>(
-    `SELECT key, label, done, done_on, sort FROM project_milestone
+    `SELECT key, label, done, done_on, sort, payment_id FROM project_milestone
      WHERE client_module_id=? ORDER BY sort, label`,
     clientModuleId,
   )
-  return rows.map((r) => ({ key: r.key, label: r.label, done: r.done === 1, doneOn: r.done_on, sort: r.sort }))
+  return rows.map((r) => ({
+    key: r.key, label: r.label, done: r.done === 1, doneOn: r.done_on, sort: r.sort, paymentId: r.payment_id,
+  }))
 }
 
-/** Tick / untick a milestone. Ticking stamps done_on (given date or today); unticking clears it. Audited. */
+/**
+ * Tick / untick a milestone. Ticking stamps done_on (given date or today); unticking clears
+ * it (and any linked payment). A payment-step tick (advance_payment / balance_payment) can
+ * pass `paymentId` to link a real payment — it must belong to the same client as this
+ * project, and its `received_on` mirrors into `done_on` unless an explicit `doneOn` is
+ * also given. Audited.
+ */
 export async function setMilestone(
-  db: DB, userId: string, clientModuleId: string, key: string, done: boolean, doneOn?: string,
+  db: DB, userId: string, clientModuleId: string, key: string, done: boolean,
+  doneOn?: string, paymentId?: string,
 ): Promise<Milestone[]> {
   return db.transaction(async () => {
     const before = await db.get<MilestoneRow>(
-      `SELECT key, label, done, done_on, sort FROM project_milestone WHERE client_module_id=? AND key=?`,
+      `SELECT key, label, done, done_on, sort, payment_id FROM project_milestone WHERE client_module_id=? AND key=?`,
       clientModuleId, key,
     )
     if (before === undefined) throw new Error('Milestone not found on this project')
     if (done && doneOn !== undefined && !/^\d{4}-\d{2}-\d{2}$/.test(doneOn)) {
       throw new Error('doneOn must be YYYY-MM-DD')
     }
-    const stamp = done ? (doneOn ?? new Date().toISOString().slice(0, 10)) : null
+    let stamp = done ? (doneOn ?? new Date().toISOString().slice(0, 10)) : null
+    let linkedPayment: string | null = null
+    if (done && paymentId !== undefined && paymentId !== '') {
+      const pay = await db.get<{ received_on: string }>(
+        `SELECT received_on FROM payment WHERE id=? AND client_id=(SELECT client_id FROM client_module WHERE id=?)`,
+        paymentId, clientModuleId,
+      )
+      if (pay === undefined) throw new Error('Payment not found for this client')
+      linkedPayment = paymentId
+      if (doneOn === undefined) stamp = pay.received_on // link date wins unless caller forced one
+    }
     await db.run(
-      `UPDATE project_milestone SET done=?, done_on=? WHERE client_module_id=? AND key=?`,
-      done ? 1 : 0, stamp, clientModuleId, key,
+      `UPDATE project_milestone SET done=?, done_on=?, payment_id=? WHERE client_module_id=? AND key=?`,
+      done ? 1 : 0, stamp, done ? linkedPayment : null, clientModuleId, key,
     )
-    await writeAudit(db, userId, 'set_milestone', 'client_module', clientModuleId, undefined, { key, done, doneOn: stamp })
+    await writeAudit(db, userId, 'set_milestone', 'client_module', clientModuleId, undefined,
+      { key, done, doneOn: stamp, paymentId: linkedPayment })
     if (done) await advanceStatusForStep(db, userId, clientModuleId, key)
     return listProjectMilestones(db, clientModuleId)
   })
diff --git a/apps/hq/test/milestone-payment.test.ts b/apps/hq/test/milestone-payment.test.ts
new file mode 100644
index 0000000..c2e6bd2
--- /dev/null
+++ b/apps/hq/test/milestone-payment.test.ts
@@ -0,0 +1,66 @@
+// apps/hq/test/milestone-payment.test.ts — payment_id on milestone + link on payment-step tick.
+import { describe, it, expect } from 'vitest'
+import { openDb } from '../src/db'
+import { seedIfEmpty } from '../src/seed'
+import { createClient } from '../src/repos-clients'
+import { createModule, assignModule } from '../src/repos-modules'
+import { setMilestone, listProjectMilestones } from '../src/repos-milestones'
+import { recordPayment } from '../src/repos-payments'
+
+async function world() {
+  const db = openDb(':memory:')
+  await seedIfEmpty(db)
+  const c = await createClient(db, 'u1', { name: 'Kothavara SCB', stateCode: '32' })
+  const m = await createModule(db, 'u1', { code: 'SMS', name: 'Bulk SMS' })
+  const cm = await assignModule(db, 'u1', { clientId: c.id, moduleId: m.id, kind: 'yearly' })
+  return { db, clientId: c.id, cmId: cm.id }
+}
+
+describe('milestone payment_id link (payment-step tick)', () => {
+  it('linking a payment to advance_payment stores payment_id and mirrors received_on', async () => {
+    const { db, clientId, cmId } = await world()
+    const pay = await recordPayment(db, 'u1', {
+      clientId, receivedOn: '2026-05-01', mode: 'bank', amountPaise: 1000_00,
+    })
+    await setMilestone(db, 'u1', cmId, 'advance_payment', true, undefined, pay.payment.id)
+    const ms = (await listProjectMilestones(db, cmId)).find((m) => m.key === 'advance_payment')!
+    expect(ms.done).toBe(true)
+    expect(ms.doneOn).toBe('2026-05-01')
+    expect(ms.paymentId).toBe(pay.payment.id)
+  })
+
+  it('rejects a payment that belongs to a different client', async () => {
+    const { db, cmId } = await world()
+    const other = await createClient(db, 'u1', { name: 'Other Bank', stateCode: '32' })
+    const pay = await recordPayment(db, 'u1', {
+      clientId: other.id, receivedOn: '2026-05-01', mode: 'bank', amountPaise: 500_00,
+    })
+    await expect(
+      setMilestone(db, 'u1', cmId, 'advance_payment', true, undefined, pay.payment.id),
+    ).rejects.toThrow(/payment/i)
+  })
+
+  it('unticking clears payment_id along with done_on', async () => {
+    const { db, clientId, cmId } = await world()
+    const pay = await recordPayment(db, 'u1', {
+      clientId, receivedOn: '2026-05-01', mode: 'bank', amountPaise: 1000_00,
+    })
+    await setMilestone(db, 'u1', cmId, 'advance_payment', true, undefined, pay.payment.id)
+    await setMilestone(db, 'u1', cmId, 'advance_payment', false)
+    const ms = (await listProjectMilestones(db, cmId)).find((m) => m.key === 'advance_payment')!
+    expect(ms.done).toBe(false)
+    expect(ms.doneOn).toBeNull()
+    expect(ms.paymentId).toBeNull()
+  })
+
+  it('an explicit doneOn wins over the payment received_on', async () => {
+    const { db, clientId, cmId } = await world()
+    const pay = await recordPayment(db, 'u1', {
+      clientId, receivedOn: '2026-05-01', mode: 'bank', amountPaise: 1000_00,
+    })
+    await setMilestone(db, 'u1', cmId, 'balance_payment', true, '2026-05-10', pay.payment.id)
+    const ms = (await listProjectMilestones(db, cmId)).find((m) => m.key === 'balance_payment')!
+    expect(ms.doneOn).toBe('2026-05-10')
+    expect(ms.paymentId).toBe(pay.payment.id)
+  })
+})