Yerushalmi (B.M. 2:5): The talmidim of Shimon ben Shetach bought a donkey for their teacher from a non-Jewish merchant and they discovered a jewel on the donkey. Shimon ben Shetach insisted on returning it. They asked, "But aviedas aku"m is permitted?" To which is answered, "Is Shimon ben Shetach a barbarian!"