fix(invoices): make self-billed invoices creditable and their dates visible (#1827)

A self-billed invoice has invoice_number null by design (the counterparty's
number lives in external_invoice_number), which broke the whole credit flow:
the confirm input was disabled and compared against null, the API minted the
literal number 'KR-null', and the credit-note PDF dropped its ML 17 kap 22
reference to the original. The editor also hid fakturadatum inside the
collapsed Forval panel, so self-billed invoices silently registered with
today's date and, being immutable, could not be corrected.

- creditConfirmNumber() falls back to external_invoice_number; the credit
  page uses it for reason default, subtitle, original row, preview, confirm
  label/placeholder/disabled, mismatch check and submit gate
- createCreditNote numbers 'KR-<external>' for self-billed originals and
  refuses with typed 400 INVOICE_CREDIT_NO_NUMBER when no number exists
- mark-sent and send select external_invoice_number and fall back for the
  credit-note PDF's reference to the original
- the Forval chip line now shows the invoice date in every mode, and
  self-billed mode renders fakturadatum + mottagningsdatum uncollapsed as
  transcription fields next to the external number

Fixes #1820


Claude-Session: https://claude.ai/code/session_01SyDuePXxUFowaPBKpAv8SF

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

app/api/invoices/[id]/mark-sent/__tests__/route.test.ts

@@ -353,6 +353,46 @@ describe('POST /api/invoices/[id]/mark-sent: PDF archival', () => {
     )
   })
 
+  // Issue #1820: a self-billed original has invoice_number null (its number
+  // lives in external_invoice_number); the credit-note PDF's ML 17 kap 22
+  // reference to the original used to be silently dropped.
+  it('falls back to the external number for the PDF reference on a self-billed original', async () => {
+    const creditNote = makeInvoice({
+      id: 'inv-4',
+      invoice_number: 'KR-SB-2026-17',
+      status: 'draft',
+      credited_invoice_id: 'inv-sb',
+      customer,
+      items: invoice.items,
+    })
+
+    enqueue({ data: creditNote, error: null })
+    enqueue({ data: company, error: null })
+    enqueue({
+      data: {
+        id: 'inv-sb',
+        invoice_number: null,
+        external_invoice_number: 'SB-2026-17',
+        status: 'sent',
+        journal_entry_id: 'original-je-1',
+        paid_at: null,
+        paid_amount: null,
+        total: 12500,
+      },
+      error: null,
+    })
+    enqueue({ data: [{ id: 'inv-4' }], error: null })
+
+    const request = createMockRequest('/api/invoices/inv-4/mark-sent', { method: 'POST' })
+    const response = await POST(request, createMockRouteParams({ id: 'inv-4' }))
+    const { status } = await parseJsonResponse(response)
+
+    expect(status).toBe(200)
+    expect(InvoicePDF).toHaveBeenCalledWith(
+      expect.objectContaining({ originalInvoiceNumber: 'SB-2026-17' }),
+    )
+  })
+
   it('fails closed and restores the draft when credit-note booking cannot start', async () => {
     const creditNote = makeInvoice({
       id: 'credit-1',

app/api/invoices/[id]/mark-sent/route.ts

@@ -160,7 +160,7 @@ export const POST = withRouteContext<{ params: Promise<{ id: string }> }>(
 
   const { data: original } = await supabase
     .from('invoices')
-    .select('id, invoice_number, status, journal_entry_id, paid_at, paid_amount, total')
+    .select('id, invoice_number, external_invoice_number, status, journal_entry_id, paid_at, paid_amount, total')
     .eq('id', invoice.credited_invoice_id)
     .eq('company_id', companyId)
     .single()
@@ -170,7 +170,11 @@ export const POST = withRouteContext<{ params: Promise<{ id: string }> }>(
   }
 
   const originalInvoice = original as CreditNoteOriginalInvoice
-  const originalInvoiceNumber = original.invoice_number ?? undefined
+  // Self-billed originals carry their number in external_invoice_number
+  // (invoice_number is null by design); without the fallback the credit-note
+  // PDF loses its ML 17 kap 22 reference to the original (issue #1820).
+  const originalInvoiceNumber =
+    original.invoice_number ?? original.external_invoice_number ?? undefined
 
   const journalEntryRequired = creditNoteNeedsJournalEntry(accountingMethod, originalInvoice)
   const isRecovery = invoice.status === 'sent'

app/api/invoices/[id]/send/route.ts

@@ -263,7 +263,7 @@ export const POST = withRouteContext(
     if (invoice.credited_invoice_id) {
       const { data: original } = await supabase
         .from('invoices')
-        .select('id, invoice_number, status, journal_entry_id, paid_at, paid_amount, total')
+        .select('id, invoice_number, external_invoice_number, status, journal_entry_id, paid_at, paid_amount, total')
         .eq('id', invoice.credited_invoice_id)
         .eq('company_id', companyId)
         .single()
@@ -273,7 +273,12 @@ export const POST = withRouteContext(
       }
 
       originalInvoice = original as CreditNoteOriginalInvoice
-      originalInvoiceNumber = original.invoice_number ?? undefined
+      // Self-billed originals carry their number in external_invoice_number
+      // (invoice_number is null by design); without the fallback the
+      // credit-note PDF loses its ML 17 kap 22 reference to the original
+      // (issue #1820).
+      originalInvoiceNumber =
+        original.invoice_number ?? original.external_invoice_number ?? undefined
     }
 
     // Preflight render: validate the PDF pipeline BEFORE consuming an F-series

app/api/invoices/__tests__/route.test.ts

@@ -625,6 +625,90 @@ describe('POST /api/invoices (create credit note)', () => {
     ])
   })
 
+  // Regression for issue #1820: a self-billed original has invoice_number
+  // null by design (its number lives in external_invoice_number), and the
+  // credit note used to be numbered the literal string 'KR-null' with notes
+  // saying 'Krediterar faktura null'.
+  it('numbers the credit note from the external number for a self-billed original', async () => {
+    const original = makeInvoice({
+      id: VALID_UUID,
+      status: 'sent',
+      invoice_number: null as unknown as string,
+      external_invoice_number: 'SB-2026-17',
+      is_self_billed: true,
+      items: [
+        {
+          id: 'item-1',
+          invoice_id: VALID_UUID,
+          sort_order: 0,
+          description: 'Provision',
+          quantity: 1,
+          unit: 'st',
+          unit_price: 10000,
+          line_total: 10000,
+          vat_rate: 25,
+          vat_amount: 2500,
+          created_at: '2026-08-01T00:00:00Z',
+        },
+      ],
+    })
+    const creditNote = makeInvoice({
+      id: 'cn-sb',
+      credited_invoice_id: VALID_UUID,
+      status: 'draft',
+    })
+
+    // Fetch original invoice
+    enqueue({ data: original, error: null })
+    // No existing credit-note draft
+    enqueue({ data: null, error: null })
+    // Insert credit note
+    enqueue({ data: creditNote, error: null })
+    // Insert credit note items
+    enqueue({ data: null, error: null })
+    // Mark creation complete
+    enqueue({ data: null, error: null })
+    // Fetch complete credit note
+    enqueue({ data: { ...creditNote, items: [] }, error: null })
+
+    const request = createMockRequest('/api/invoices', {
+      method: 'POST',
+      body: { credited_invoice_id: VALID_UUID },
+    })
+    const response = await POST(request)
+    const { status } = await parseJsonResponse(response)
+
+    expect(status).toBe(200)
+    const [invoiceInsert] = findCall('invoices', 'insert') ?? []
+    expect(invoiceInsert).toMatchObject({
+      invoice_number: 'KR-SB-2026-17',
+      notes: 'Krediterar faktura SB-2026-17',
+    })
+    expect((invoiceInsert as { invoice_number: string }).invoice_number).not.toContain('null')
+    expect((invoiceInsert as { notes: string }).notes).not.toContain('null')
+  })
+
+  // Defensive path: both numbers null cannot happen for an issued invoice
+  // (DB constraint), but a garbage 'KR-null' must never be minted.
+  it('returns a typed 400 when the original carries no number at all', async () => {
+    const original = makeInvoice({
+      id: VALID_UUID,
+      status: 'sent',
+      invoice_number: null as unknown as string,
+    })
+    enqueue({ data: original, error: null })
+
+    const request = createMockRequest('/api/invoices', {
+      method: 'POST',
+      body: { credited_invoice_id: VALID_UUID },
+    })
+    const response = await POST(request)
+    const { status, body } = await parseJsonResponse<{ error: string }>(response)
+
+    expect(status).toBe(400)
+    expect((body.error as unknown as { code: string }).code).toBe('INVOICE_CREDIT_NO_NUMBER')
+  })
+
   it('returns an existing credit-note draft instead of creating a duplicate', async () => {
     const original = makeInvoice({ id: VALID_UUID, status: 'sent' })
     const existing = makeInvoice({

app/api/invoices/route.ts

@@ -301,6 +301,17 @@ async function createCreditNote(
     })
   }
 
+  // Self-billed originals have invoice_number null by design (the DB
+  // constraint invoices_self_billed_numbering enforces it); their number
+  // lives in external_invoice_number. Without this fallback the credit note
+  // would be numbered the literal string 'KR-null' (issue #1820). Both null
+  // is impossible for an issued invoice, but refuse defensively rather than
+  // mint a garbage number.
+  const originalRef = originalInvoice.invoice_number ?? originalInvoice.external_invoice_number
+  if (!originalRef) {
+    return errorResponseFromCode('INVOICE_CREDIT_NO_NUMBER', log, { requestId })
+  }
+
   // Returning the existing credit note makes the action idempotent. A
   // cancelled, unissued draft is reopened so the deterministic KR number can
   // be reused without colliding with the company-wide invoice-number key.
@@ -325,7 +336,7 @@ async function createCreditNote(
           status: 'draft',
           invoice_date: today,
           due_date: today,
-          notes: input.reason || `Krediterar faktura ${originalInvoice.invoice_number}`,
+          notes: input.reason || `Krediterar faktura ${originalRef}`,
           updated_at: new Date().toISOString(),
         })
         .eq('id', existingCreditNote.id)
@@ -354,7 +365,7 @@ async function createCreditNote(
     return NextResponse.json({ data: maskEmbeddedCustomer(existingCreditNote) })
   }
 
-  const creditNoteNumber = `KR-${originalInvoice.invoice_number}`
+  const creditNoteNumber = `KR-${originalRef}`
 
   const { data: creditNote, error: creditNoteError } = await supabase
     .from('invoices')
@@ -389,7 +400,7 @@ async function createCreditNote(
         : 0,
       deduction_personnummer_encrypted: originalInvoice.deduction_personnummer_encrypted ?? null,
       deduction_personnummer_last4: originalInvoice.deduction_personnummer_last4 ?? null,
-      notes: input.reason || `Krediterar faktura ${originalInvoice.invoice_number}`,
+      notes: input.reason || `Krediterar faktura ${originalRef}`,
       credited_invoice_id: input.credited_invoice_id,
       // Copy the original's dimension bag so the credit-note verifikat nets
       // against the same dimension cells in reports (dimensions PR7).