Hadith 51
Chapter 17: Zakat - كتاب الزكاة
Yahya said that Malik was asked about whether a man who gave some sadaqa, and then found it being offered back to him for sale by some one other than the man to whom he had given it, could buy it or not, and he said, "I prefer that he leaves it."
Reference Muwatta Malik 51
USC-MSA web (English) reference : Book 17, Hadith 51