Every trustee may retain or deduct out of the goods, effects and credits in his hands all his demands against the principal defendant, of which he could have availed himself if he had not been summoned as trustee, by way of counterclaim on trial or by a setoff of judgments or executions between himself and the principal defendant, except unliquidated damages for wrongs and injuries. He is liable for the balance only, after their mutual demands are adjusted.