Shaykh Muhammad al-‘Uthaymeen (may Allaah have mercy on him) said: 

Whoever pays on behalf on one whose zakaat al-fitr he is not obliged to pay must have his permission. So if Zayd pays it on behalf of ‘Amr without his permission, that is not valid, because Zayd is not obliged to pay zakaat al-fitr on behalf of ‘Amr. It is essential that the right intention (niyyah) be formed, either on the part of the one who is obliged to pay it or on the part of the one who pays it on his behalf. This is based on a principle that is well known to the fuqaha’, which they called tasarruf al-fudooli, referring to the question of when a person acts on behalf of another without his permission, is the action completely invalid or does it depend on the permission and approval of the other person? 

There is a dispute among the scholars on this matter. The most correct view is that it is valid if the other person gives his approval. And the Shaykh quoted the hadeeth of Abu Hurayrah’s interaction with the Shaytaan when he was guarding the zakaah. See the text of this hadeeth in the answer to question no. 6092. This is corroborated by the fact that the Messenger (peace and blessings of Allaah be upon him) approved of this action on the part of Abu Hurayrah and regarded it as valid, even those what was taken from was zakaah, and Abu Hurayrah was appointed to guard it, and not anyone else. 

Al-Sharh al-Mumti’, 6/165. 

And Allaah knows best.