Key legal question
Whether the defendant could validly set off his claim against X. against the assigned cost award owed by the defendant.
Extracted holding
Yes. The mutuality requirement for set-off was met because the cost award was held by the original co-creditors as solidary creditors, so payment to X. would have had discharging effect.
Extracted reasoning
A solidary creditor relationship existed by contractual agreement between the parties to the earlier litigation. As a result, each creditor could demand the whole performance and the debtor could perform validly to either creditor; set-off against X.'s claim therefore extinguished the debt.