Ibn Jurayj (may Allah have mercy on him) says: I said to Ata' (may Allah have mercy on him): I made a contract of manumission (mukatabah) with two men in a sale on the condition: "The living one of you is responsible for the dead one of you, and the wealthy one of you for the destitute one of you." He said: This is permissible. This is also the view of Amr bin Dinar (may Allah have mercy on him) and Sulayman bin Musa (may Allah have mercy on him), and he said: It is za'amah, meaning hamalah (suretyship).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب المكاتب / 21633