Mujahid (may Allah have mercy on him) said that Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) borrowed some dirhams from a man, then repaid him with better ones. The man said: "O Abu Abd al-Rahman! These dirhams are better than my dirhams." Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said: "I knew that, but I have paid them willingly."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10944