Welcome to today's economy

Subject A wants to borrow some Gold from Subject B. The latter agrees to it, but wants to make sure he gets it back someday, so he writes on a paper "A owes me X amount of Gold".

However, B would want something in exchange, right? B cannot expect to get back more than X gold, but A can give B another paper that says "C owes me Y amount of Silver".

And welcome to today's economy. We cannot expect to get our favours back, so we trade papers that unite us all in a complicated but beautiful economic system.