Possibly in Japanese do not exist therm like this. Even in English it could be rare. Anyway, how could I translate it in Japanese? The meaning of sub-debtor should be "debtor of debtor".
Let me give the example sentence (please sorry in case if you will find it confusing):
"The pledge of receivables arising from the Respective Receivables created under the Pledge Agreement shall be considered as validly existing until the moment the Sub-debtor receives a notice from the Pledge on termination of such pledge."