[The Ruling on Partnering with a Zakat Official Who Buys a Commodity with Zakat Funds]

The answer—and Allah is the granter of success and aid—is that the solution and lawful workaround is: you and your brother purchase the commodity for a price on credit, not specified in particular cash. On that basis, the commodity is lawful for you, even if your brother later pays the price from zakat funds. Your brother alone bears responsibility for his disposition of the zakat and for its misapplication.
As for the seller, what appears to me—and Allah knows best—is that it is permissible for him to take the price even if he suspects it is from zakat funds. That is because when the purchaser disposes of the zakat monies in his possession, by that act he becomes liable for what he has disposed of; when later called to account for the zakat, he will be ruled liable to pay the like of what he consumed, not the seller who received the price.
The jurists of the madhhab have said: He is not obliged to return the very item once it has left his hand, but rather Ê its like. This indicates that the one into whose hand the money has passed (i.e., the seller) is neither liable nor responsible for it; rather, the responsible party is the one who took the zakat out of his possession. What reaches the seller in that case is not zakat, and the one answerable for the zakat is the owner upon whom it was due, or his agent who was entrusted with it.
If the owner knew the dishonesty of the official charged with the zakat, then what he handed over to him does not count as zakat; thus the owner remains one who has not discharged his zakat.
If the owner believed him to be just and trustworthy and that he would place the zakat in its proper outlets, but later discovered the contrary, then the obligation in that case upon the owner is to pay the zakat anew and discharge it a second time.
If nothing becomes apparent to the owner, then perhaps it will suffice him, for Allah Most High has said: “There is no blame upon you for what you do by mistake, but [only] for what your hearts intended.” [Al-Ahzāb:5]
The evidence for what we have mentioned is analogy: if a man owes debts and then sends funds with messengers to settle those debts, but the messengers are negligent with the money, dispose of it, and betray the trust, then the man who sent the money remains liable for the debt, and that remittance does not absolve him of the debt in any way.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2