Will the wholesale merchant be absolved of responsibility if he asks the retail merchant to pay the zakat remaining from his debt? And is it permissible for the retail merchant to account for zakat by writing off the debts of his insolvent customers without informing them or the owner of the money?

It is permissible to appoint an agent for the disbursement of Zakat, whether the owner of the wealth appoints someone to disburse the Zakat from the principal's money, or appoints someone to disburse it on his behalf from the agent's money, and then the principal reimburses him. However, it is not valid to appoint someone to disburse Zakat from a debt owed to the agent, as this does not absolve the wealth owner's responsibility. Instead, he must disburse the Zakat himself, or collect the debt and then appoint someone to disburse it, or appoint someone to disburse it after collecting the debt. Furthermore, it is not permissible for the agent to count what is owed to him by those in hardship as Zakat and thereby absolve their debts.