Riba In Mubadalah

Riba is a most mysterious subject. Even to our great predecessors it proved to be a very enigmatic prospect. Where does Riba manifest itself? How? Where does one draw the line? How does one know whether a transaction is Riba, or just looks like it?
Sometimes it is clear, and other times it is like being in a hall of mirrors. One wonders whether it is a true reflection, or just an illusion.
It is an action that practically every human has done in his lifetime. Changing money. Not the changing into another currency, but getting change for larger bills.
What rules govern this type of transaction? What are the pitfalls of which to be careful? What are the views and disputes of scholars?