Why did Uthman, may Allah be pleased with him, return the Zakat money when Ali, may Allah be pleased with him, sent it to him?

Uthman's—may Allah be pleased with him—response was not concerning the zakat (definition) money, but rather the book of Sadaqat (charitable donations) that Ali—may Allah be pleased with him—sent to him. Uthman was knowledgeable in matters of Sadaqat, so he did not see a need for that book. Ali—may Allah be pleased with him—did not object to that. It is possible that Uthman did not find the complaints of the collectors to be substantiated, or that wisdom dictated delaying the rejection, or that the matter was among the recommended acts, not the obligatory ones. Therefore, Ali did not mention it in a negative light.