When it is a question of money, everybody is of the same religion.

People differ loudly about most things and then move in remarkably similar ways once money is involved. Stated principles are cheap until they cost something, and the moment they cost something is where you learn what a person actually holds. This is worth knowing without becoming cynical; it is a description of people under pressure, not an accusation.

In your next arrangement with someone, watch what happens when the numbers get uncomfortable rather than what was promised when they were easy. Write the terms down while everyone is still generous. Then hold yourself to the same test you are quietly applying to them.