Is it permissible for me to use zakat money in my possession to purchase a personal item, then return its value immediately upon my return home, knowing that I have its value available to me?

If someone is given money as zakat (definition) or sadaqah (definition) to distribute, it is a trust (amanah (definition)), and it is not permissible for them to take from it with the intention of returning it, except with the permission of the giver. This is because they are a trustee agent (wakeel mu'taman) who acts with the permission of their principal. If they do so without permission, they are betraying the trust. Ibn Qudamah said: "The agent (wakeel) does not possess the right to act except within what the permission of his principal dictates." Ibn Uthaymeen said: "If he is given money as a trust... it is not lawful for him to dispose of it in any way." If this has occurred, there is hope in Allah's forgiveness, and in the future, it is obligatory to dispose of the trust within the limits of the permission or after seeking permission from its owner.