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