A kind of sale which is prohibited. Mishkat, book xii. ch. v. pt. 2: “The Prophet has forbidden selling on credit for credit.”
‘Abdu ‘l-Haqq explains it thus: “If ‘Amr owe Zaid a piece of cloth, and Bakr ten dirhams, and Said say to Bakr, I have sold you the piece of cloth, which is with ‘Amr for ten dirhams” – this sale is forbidden.
Based on Hughes, Dictionary of Islam