Can we release just one of the remaining order lines?

No. Release covers every line that is not already delivered, so you cannot choose which of the remaining lines it applies to. Three constraints explain the behaviour: only one release is allowed per order, a release cannot cover items that are already delivered, and all existing deliveries must be in a non-incomplete state before a release can start. The Release option only becomes available once at least one delivery has completed and the order is not yet fully delivered. If part of the remainder will still ship, create a delivery for those lines when stock arrives and release only what will not ship. If the customer changes their mind about something already delivered, that is a refund, not a release.