Put it on the receiving record as an outstanding ordered spare, with the quote line attached. Don't let 'main kit delivered' become 'everything delivered'. Has the seller actually agreed the extra cable is owed?
Ask them for its actual status and a way to identify your parcel when it leaves. 'With another shipment' tells you nothing about whether your cable is even packed yet.
Tell maintenance that too, because 'spare on its way' and 'spare not available yet' lead to rather different plans when the fitted one needs replacing.
Right. Keep the stock admission with the order record, so the next person chasing it doesn't spend another week trying to track a parcel that never left.