Sayyiduna Ma'qil bin Yasar (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "Whoever buys a slave in order to set him free should not put the condition of freeing him before his owners, for it is a contract (or deal) relating to provision."
Al-Tabarani recorded it in al-Mu'jam al-Kabir and al-Mu'jam al-Awsat; its chain contains Sa'id bin al-Fadl al-Qurashi, whom Abu Hatim declared da'if (weak) while others declared him strong. As for the narrator Abu Abdullah al-Anazi, I have not found anyone giving his biography.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب البيوع / 6391
Hadith Gradingمحدثین:إسنادہ ضعیف
Hadith Takhrij«اخرجه الطبراني فى المعجم الكبير (224/20) ، أخرجه المعجم الأوسط للطبراني برقم: 5360»