When one enters a gas station (in Israel), he is greeted by a large sign offering a (large) discount for diesel if you pay cash. Technically this would be ribis according to the gemara. How is it such a common practice if it seems to be assur? Is there a heter (other than making a heter iska with the gas station)
In general there is a serious problem when two different prices are offered -- one for cash and one for payments. For a comprehensive discussion of this problem, see Bris Yehudah (Rav Y. Blau). However, the case of the gas station might pose less of a problem, because it is not actually the gas station which is extending the credit. When you give a credit card the station has finished his claim with you, and in essence is extending credit to the credit card company, and the credit card company is extending the credit to you. Therefore, the higher price is not for the credit, but rather for paying with "better money." These are just my thoughts, though; for a practical ruling a competent rabbinical authority should be consulted.
D. Zupnik