All perfect praise be to Allah, prayer and peace be upon Prophet Muhammad and upon his family and companions.
Intention is a pillar of Zakah (obligatory charity), so it (Zakaat) must be paid only as Zakah because it is not permissible to perform obligatory and voluntary acts of devotion with one intention. Therefore, paying it with the intention of Sadaqa (voluntary charity) and Zakah is tantamount to paying it as Sadaqa and being rewarded for it as a voluntary act of devotion, consequently; it must be repaid ,as Zakah, since the validity of intention is a condition for obligatory solely for obligatory acts of devotion alone, as mentioned in the [Al-Majmoo`”] by Al-Imam An-Nawawi: "If he/she paid five dinars with the intention of performing the voluntary as well as the obligatory acts of devotion, he /she will be rewarded for the voluntary. And Allah knows best.