Answer: A Muslim must first be certain that the person to whom he is giving Zakat al-Fitr is genuinely in need and entitled to receive it. Once it is clear that the individual is poor and deserving, he may hand the alms over to him and is not held responsible for whatever the recipient subsequently does with it.
The recipient is free to sell the grain, give it away as a gift, eat it himself, or even distribute it as charity on his own behalf. The giver incurs no blame so long as—according to his best judgment—the recipient was eligible. But if the giver knows that the person is not eligible, that he is in fact well-off, then it is unlawful to give it to him. Seeing people in the street asking for Zakat al-Fitr is a sign of need, yet the giver should still verify their circumstances; and if he learns of someone whose need is greater, he ought to direct the alms to the one in greater need.
Purchasing the alms back from the recipient is not allowed. Anyone who has donated something—whether Zakat on wealth, Zakat al-Fitr, or any other form of charity—may not buy it back or reclaim it after giving it.
Question: Perhaps the inquirer intends to ask: if he sees a needy person take Zakat al-Fitr in one place and then go elsewhere to sell it, may he buy it from him so that he can in turn give it as his own zakat?
Answer: He may not purchase the very charity he himself gave to that person. As for buying Zakat-items that others have given to him, there is no objection; that is permissible.