MC doesn’t work that way. They get a flat rate right off the top. He cuts prices by 15 cents, and MC still gets their cut. He’ll end up losing money even more.
If they are cash purchases, Master card doesn’t have anything to do with it.
They get a flat rate on all gas sales, regardless if people don’t use their MC on a transaction and pay with cash? WTF?? Why would a station owner agree to that? You’d never see that kind of deal at a restaurant, clothing store or anywhere.