Is it obligatory to retain the value of Zakat that was set aside but delayed in reaching the poor person, or is it permissible to pay it from other funds upon meeting the poor person?

The money your friend paid to the poor and needy on your behalf is considered a debt upon you that must be repaid when circumstances allow. It is not necessary to set it aside by itself; there is no benefit in doing so unless it is handed over to its owner or his representative. A Muslim is required to write down his will and his debts to ensure rights, as the Messenger of Allah, peace and blessings be upon him, said: "It is not fitting for a Muslim man who has anything to bequeath that he should sleep for three nights without having his will written down with him."