Question:

One person gives another person notes of zakat or any other wajib sadaqah to pass on to the poor. This agent changed the notes. For example, he took ten R10 notes and exchanged it with a R100 note. Is it permissible to do this or is it necessary to give the poor the exact notes received?

Answer: باسم ملهم الصواب

Zakat will in all cases be considered paid. However, the validity of changing the notes depends on the explicit or implicit permission of the one commissioning the agent (mu-ak-kil). Nowadays the norms is that there is such a permission. Therefore there is no need for an explicit permission. Yet it is best to aquire an explicit permission.

(Mufti Azam) Rashid Ahmed Ludhyanvi (May Allah have mercy on him)

Ahsanul Fatawa Vol.4 Pg.299-300
Also see here.