Write off a vendor bill balance

Writes off the outstanding balance of a vendor bill as income recognition.

GAAP basis (FASB ASC 405-20): writing off AP is a gain from extinguishment of liability — NOT an expense. The journal entry posted is:

DR  Accounts Payable (ap_account)         <balance_due>
CR  Vendor Write-Off Income (4998)         <balance_due>

Distinction from DELETE /vendor-bills/{id} (void):

• Void reverses the original AP entry entirely and requires amount_paid = 0.
• Write-off posts a new income-recognition JE and allows partial payments to remain. Only the remaining balance_due is written off.

The bill’s status transitions to written_off (terminal state). The original bill journal entry remains intact on the books.

Idempotency: re-POST with the same (source_type=WRITE_OFF, source_id=bill_id) pair returns the cached result without re-posting the JE.

Period lock: write-off of a bill in a locked tax year returns HTTP 422 closed_period.

Requires purchasing:write scope.