All perfect praise be to Allah, The Lord of The Worlds, and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
Wakalah (authorization) is when someone authorizes someone else to perform a lawful task on his behalf, so he can buy and sell, but not as if it is for himself (authorized); therefore, his actions must be for the benefit of the authorizer. Moreover, there is a consensus among Muslim scholars that the authorized is entrusted with achieving the benefit of his authorizer.
Prophet Mohammad(PBUH) said: " Pay the deposit to him who deposited it with you" {Abu Dawoud and Tirmizi}. An-Nawawi (May Allah be pleased with him) said: "Wakala takes the ruling of a trust since the authorized is trustworthy, and he isn`t required to give a compensation, except in case of misconduct and negligence." {Rawdat At-Talibeen, 4/325}.
Moreover, Al-Imam Ar-Ramli (May Allah be pleased with him) said: "A guardian does what is beneficial to the one under his guardianship and an authorized does what is beneficial to the one who had authorized him" {Asna-Al-Matalib, 2/46}.
Accordingly, if the authorized acted against the benefit of the authorizer, then his action is null and void. Some examples of such acts are: non-adherence to the conditions of the authorizer, or selling against deferred payment without his approval or selling with grave deception, and the like." {Moghni Al-Mohtajj, 3/243}. And Allah knows best.