Sayyiduna Abdullah bin Amr bin al-As (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) ordered him to equip an army. Abdullah bin Amr (may Allah be pleased with them both) said: We had no mounts. He (peace and blessings be upon him) said: "Buy mounts (to be paid for) when the sadaqah camels arrive." So Abdullah bin Amr (may Allah be pleased with them both) bought one camel for two camels, (payable) when the sadaqah camels arrived, by the order of the Messenger of Allah (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10529