Secret profit
When an agent deals with a counterparty on its principal’s behalf, the agent must account for the transaction proceeds to the principal. The agent cannot make a secret profit from the principal’s assets.
For example, say I gave you £50 to run up to the super Black Gull Bookshop in East Finchley to buy a copy of the gorgeous Folio Society edition of Thomas Kuhn’s The Structure of Scientific Revolutions which I saw in the window for £50.
If, when you get there, the book is on special, and you manage to buy it for £30, you don’t get to keep the £20 change, even though I was quite prepared to pay £50 for the book. You must account to me for the £20. Seems obvious, right?