Sayyiduna Malik bin Aws bin al-Hadathan said: I was seeking to exchange a hundred dinars. Sayyiduna Talhah bin Ubaydullah (may Allah be pleased with him) called me, and we agreed on the price between us until he bought from me; he took the gold and was turning it over in his hand. Then he said: Wait until my treasurer comes from the place called al-Ghabah. Sayyiduna Umar (may Allah be pleased with him) was listening, and he said: Do not part from him until you receive the price from him. Then he said that the Messenger of Allah (peace and blessings be upon him) said: "Gold for gold, silver for silver, wheat for wheat, dates for dates and barley for barley is usury (riba), except in hand-to-hand exchange."
(b) In the hadith of Imam al-Shafi'i (may Allah have mercy on him) it is: until my treasurer or my slave-girl returns from the place called al-Ghabah.
Imam al-Shafi'i (may Allah have mercy on him) said: I read it correctly before Imam Malik (may Allah have mercy on him), and I had no doubt. Then time passed and I did not retain it well, so doubt arose in me whether it was "my treasurer" or "my slave-girl"; but others besides me narrate only the word "treasurer".
(b) Abdullah bin Yusuf mentions from Imam Malik (may Allah have mercy on him) only the word "treasurer".
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10474