Creates a bill of type sale, refund, void, layaway or foc (free-of-charge). Requires the p/c-bill permission on the bill location (voids at non-restaurant locations require p/d-bill). Pass is_cloud_settle: true to book the bill against the location's cloud register — the invoice number is then assigned server-side when number is omitted. Without is_cloud_settle a valid open register_session.id is required. The client is the source of truth for amounts: line and bill totals are stored as sent. A bill with no payments stays outstanding (payment_status: payment-pending) and can be settled later via the Payments endpoints. For refunds set parent_id to the original bill and send negative quantities/totals; the refund total must be negative. Note: the settled_at key must be present in the payload (JSON null is fine for an unsettled bill) — omitting the key entirely fails.