Link or unlink what an invoice bills

Attach an existing invoice to the pay application, payment-schedule installment and/or bid it bills, or detach it (send null). Allowed in every status except voided, so an imported, manual or already-paid invoice can be linked after the fact; only the link columns and the derived sourceType change — never lines, totals or status — and nothing is pushed to QuickBooks. The pay app must be on the invoice's own project, live (submitted, sent, approved or paid) and not already claimed by another live invoice; the installment must be on the project's bid's payment term and not billed by another live invoice. An invoice bills a pay app or an installment, not both. The response carries the project's billed figures before and after; dryRun: true returns them without saving.