[When a Poor Person Appoints as Agent a Man to Whom Zakat Is Not Lawful and Authorizes Him Regarding It]

Answer—and Allah is the Granter of success: Allah the Exalted said, “Zakah expenditures are only for the poor and the needy …” [al-Tawbah:60]. Thus, zakat is not lawful for anyone other than those whom Allah has mentioned in this verse, “except for a man who purchases it with his own wealth …” (hadith).
And Allah the Exalted said, “Cooperate in righteousness and piety, but do not cooperate in sin and aggression.” [al-Māʾidah:2]. What is mentioned in the question is not cooperation in righteousness and piety; rather, it is cooperation in consuming the wealth of the poor and needy, and in depriving them of the zakat that Allah has made obligatory for them.
Such agency is not valid; indeed, it is a void agency, for it entails that the poor person has appointed the wealthy person to consume from zakat what Allah has forbidden to him.
In reality, the poor person sells to the rich man the mere semblance of an agency for a fixed or unknown price—or the rich man contrives a stratagem for the poor person until he fashions for him the semblance of an agency.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1