Pay with your promise!
What is a virtual Promise and how can it be explained? Nothing is easier because it's exactly like a Promise in the real world. A Promise is a Promise and shall not be broken. If you Promise to do something... do you think you should you keep it? It starts with your Promise to bring something back you borrowed. Will you keep that Promise?
We believe that most people are good and honest in the real world. Why should it be different in the virtual world?