Sayyiduna Abu Hurayrah (may Allah be pleased with him) says: The Prophet (peace and blessings be upon him) owed a man a camel of a certain age (in its third year). The man came to claim his due, and he (peace and blessings be upon him) said: "Give it to him." The Companions searched but found only one older than it. He (peace and blessings be upon him) said: "Never mind, give him this camel." The man said: "You have repaid me in full; may Allah repay you in full." He (peace and blessings be upon him) said: "The best of you are those who are best in repaying."
Imam al-Shafi'i (may Allah have mercy on him) says: This hadith is established from the Messenger of Allah (peace and blessings be upon him), and this is my position too. In this hadith it is stated that the Prophet (peace and blessings be upon him) made the transaction for a camel by its description. This shows that such a transaction may be made for all animals, that is, it is said: "I am taking from you a camel in its third year," so when I give back, I will give back a camel in its third year. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 11098