Kernrechtsfrage
Whether the defendant could validly set off his claim against X. against the assigned cost award owed by the defendant.
Extrahierter Entscheid
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.
Extrahierte Begründung
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.