הלכה על אבות 5:10

The Mishnah (Avot 5:10) condemns individuals who act in the manner of Sedom by claiming, “What is mine is mine, and what is yours is yours.” This implies that one should not always demand from another exact payment to which one is entitled. Thus, one should not demand payment for gas and tolls from another who has asked him to ride home in his automobile if the latter did not take the former out of his way. Despite the fact that the rider has saved money, since he does not have to pay transportation costs, it is nonetheless Somodite behavior for the driver to demand payment if he has lost nothing. In fact, the Ri (presented in Tosafot ad. loc.) implies that the rule of kofin al middat Sedom is a Torah level law and not merely a rabbinic enactment.