Buying zakaah items after giving them to the poor person
Question

Some shopkeepers give the zakaah of their wealth in the form of milk and rice, and after giving it to the poor person they buy it back from him for something less than the real price. Is this permissible?.
Praise be to Allah.It is permissible to give the zakaah on trade goods in the form of trade goods. Please see the answer to question no. 22449. 
The scholars (may Allah have mercy on them) differed as to whether it is permissible for the one who gives charity to buy back what he gave for the sake of Allah (whether it was zakaah or sadaqah) or not. 
The Hanbalis are of the view that it is not permissible, because of the report narrated by al-Bukhaari (1490) and Muslim (1620) from ‘Umar (may Allah be pleased with him) who said: I donated a horse (to be ridden in jihad) for the sake of Allaah, and the one who had it neglected it. I wanted to buy it and I thought that he would sell it for a cheap price. I asked the Messenger of Allaah (blessings and peace of Allah be upon him) about that. He said: “Do not buy it and do not take back your charity, even if he gives it to you for a dirham, for the one who takes back his charity is like the dog that goes back to its vomit.” 
And because buying it is a means of taking back some of it, because the poor person will feel shy before him, so he will not bargain with him over its price, and he may reduce the price hoping that he will give him more charity. Or perhaps he knows or thinks that if he does not sell it to him, he will take it back from him and that scenario should be avoided, such as if he stipulates that he should sell it to him.
End quote from al-Mughni, 2/271 
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked: Is it permissible for a rich person to buy his zakaah back from the poor person to whom he gave it? 
He replied: It is not permissible. ‘Umar ibn al-Khattaab donated a horse of his (to be ridden in jihad) … Then he quoted the hadeeth referred to above. The reason is that if the sheep that he gave to this poor person is worth one hundred, he will sell it to him for eighty in the hope that he will give to him again the following year, because he was easy-going with him.
End quote from al-Sharh al-Kaafi
See also: Majmoo‘ Fataawa Ibn Baaz (20/67) and al-Lajnah al-Daa’imah, 13/142 
And Allah knows best.