Imam al-Shafi'i (may Allah have mercy on him) said: A shaykh from the people of Makkah told me: I heard Tawus (may Allah have mercy on him), while I was standing by him, as he was asked about selling zakah before it has been received. Tawus (may Allah have mercy on him) said: By the Lord of this House! It is not permissible to sell it before it is received, nor after it is received.
Imam al-Shafi'i (may Allah have mercy on him) said: This is because the Messenger of Allah (peace and blessings be upon him) said: "It is to be taken from the rich and given to the needy." The needy and the poor are to be given the thing itself, not its price.
The Shaykh said: The reports that have come concerning the obligatory zakah are evidence in this matter.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7628
Kulthum narrates from his father (may Allah be pleased with him) that they offered the Prophet (peace and blessings be upon him) to buy from him the remainder of the zakah that was still due from them. He (peace and blessings be upon him) said: "We do not sell anything of the zakah until we have taken possession of it."
This chain is not strong. It is also narrated from Isma'il bin Ayyash through two chains of his, forbidding that: from Abu Sa'id as munqati' (broken), and from Abu Hurayrah as connected and marfu'; and Isma'il is not used as proof.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7629
Muhammad bin Rashid (may Allah have mercy on him) narrates from Makhul (may Allah have mercy on him) that the Messenger of Allah (peace and blessings be upon him) said: "Do not buy the zakah charities until they have been marked and taken into possession."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7630