Alms Tax: Priority of Zakat, Khums, and Other Obligations
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 2000
If a person owes the one-fifth tax (khums) or zakat, and recompense (kaffārah), vow (nadhr), and suchlike are also obligatory for him, and he has debt as well, in the event that he cannot pay all of these obligations and the wealth on which khums and zakat are obligatory has not perished, he must pay the khums and zakat. If it has perished, then paying zakat, khums, and settling his debt has priority over kaffārah and nadhr.
-Ayatullah Sistani, Practical Laws of Islam, Alms Tax (Zakat)