Alms Tax: Purchasing Produce Subject to Zakat
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1886
If a person buys wheat, barley, dates, or grapes, and he knows that the seller has given zakat on them, or he doubts whether the seller has given zakat on them or not, then zakat is not obligatory for him; but if he knows that the seller has not given zakat on them, he must give it. However, if the seller has cheated him, he can claim the amount of zakat he gave from the seller after giving the zakat.
-Ayatullah Sistani, Practical Laws of Islam, Alms Tax (Zakat)