If someone gives you an IOU, hold onto it and cash it in when you can!
This is a great story about John McDonagh, a cab driver in New York City who gave a ride to Shane Gaffney in 2013. Shane didn't have any money to pay for it, but he did give John an IOU for two pints of Guinness at his family's pub in Dublin.
Fast forward 9 years, now John is touring his one man show about being a taxi driver, Off the Meter, On the Record, and Dublin was on the schedule. He brought the IOU to the Gaffney and Son Pub in Dublin, which Shane's family does own. He got his two pints, along with a warm welcome and a celebration!