I'm sure that the people living in mars will be homesick eventually and start to trade in memorabilia items. They might even use US dollars just to remind them of earth
US Dollar can be used in Mars, as long as physical cash is the format. I don't think that we'll be able to use online modes such as Paypal and Visa anytime soon due to data transfer issues. That said, there is a strong chance that any upcoming colonies in Mars will be much tempted to print their own currency rather than depending on the currencies that are being used here. Gold is another asset that can be used as cash.